I'll go ahead and respond to your post, and then give you my thought process for how I got here, experience, blah blah blah. It might be kind of a long post.
I've heard (read) great things about the Screamy (on MTBR, if it's to be trusted), and honestly the concept of the "all mountain 29er" or whatever it's being billed as is incredibly interesting to me.
These are my thoughts on the Stache also. I demoed one the other day, and other than feeling a little stretched out, it was a fine bike.
I live in Michigan, but I hate the riding here. I hate it so much that when I moved back, I sold my mountain bike and got a road bike. It didn't make sense to have two bikes when I was back in school and not working, but now that I'm headed back to work, it's a lot easier to justify the expense of another bike. This will be a traveling bike for me.
Quick answers: I live in Michigan. The trails look like flat sand. I like to ride Park City and Fruita/Moab. I like flow, but I'm fine with square edged 1 foot rocks/drops/etc., gnar ain't so bad in a general sense, but I started riding too late in the game to really appreciate being airborne; put simply, I'm not young or stupid enough to learn how to huck.
Also, a little background: I started mountain biking while I was living in Park City. I thought it'd be ok, you know, something to do in the summers when I wasn't skiing, but frankly I fell in love with the riding out there. I was rolling a Gary Fisher Marlin (with the original genesis geometry), and had a blast with a 100mm travel hard tail out there. Again, I wasn't hucking anything, and I'm not entirely sure I want to, but that's where I learned to ride. Made it out to Fruita and Moab, both of which I loved. To me, perfection is Horse Thief's Bench in Fruita (again,
bike21 know the trail).
So now I'm looking at getting another ride. Like I said briefly, my options are a low end (read as: heavy) all mountain bike (5-6" of travel) that I KNOW I can take anywhere, but won't do any one thing very well, or start diversifying my quiver of bikes. I've come to the conclusion that there's no right or wrong answer here, both avenues are reasonable ways of approaching this problem. Frankly, the issue is really whether I start getting specialized bikes for specialized tasks now, or do it when we move back to Utah in 3-5 years.
My thoughts on the all mountain type rigs were initially this: grab something generic like a Santa Cruz Heckler, ride it stock until I beat the crap out of it, and deal with hauling 30-32 pounds of a not-very-efficient-at-climbing bike around until I get back to Utah. At that point, if I find that I want to get into the park, I can replace the rear shock with a coil shock, and then put maybe 170mm of travel up front (and try to break the frame), or I can put a 140mm fork up front and get the rear shock PUSH'd in a few years. It's not a bad move, in my mind. A bike like this would be able to do nothing well, but could get around on any trail. The drawbacks are obvious; weight and inefficiency. It would be brutal to try to do a stage race on something like this (which is something I think I'm interested in trying my hand at).
My other thoughts were the Stache. You know the benefits and the drawbacks, so I won't waste my time with a discussion of the details of the bike. It's a great idea, and I found it's quite a nice ride.
So fast forward to a few weekends ago where I was finally able to get on some of these bikes at the local demo day. I was able to ride a Giant Reign, Trek Rumblefish Pro, Trek Remedy 9.8 carbon, Trek Stache 8, and a Specialized S-Works Stumpjumper FSR 29er. Here's the quick and dirty of what I thought of each of these bikes:
Giant Reign: Fine bike, but creaky as hell. The CTD ("new and improved" pro pedal) wasn't setup properly for this bike, so the shock was riding full open on flat XC trails, so that sucked some of the efficiency out of the rear shock. Overall, there's something I've never liked about Giant bikes, and there's still something I just don't like about Giant bikes. Other folks love them, but there's something about the suspension design that I can't dig, and I don't know what it is. Good bike for somebody else.
Trek Rumblefish Pro: I came into this demo day being a 29er hater. I rode a 29er once 5 years ago and it was horrible (Gary Fisher Hi-Fi...the first one they produced). So when the guys told me to check this one out, I wasn't very interested, but I thought I'd give it a go. I have to admit, I'm a changed man now. The Rumblefish's low speed steering was quicker than I expected, and the thing ate up any terrain (roots) in front of it without seeming to notice it was there. Climbed well, accelerated well, and frankly, I couldn't find any draw backs to the bike. The only thing I'm worried about is catching air with this bike, what with 28 spoke wheels and giant 29er rims. I could see how it could be really easy to destroy those wheels if you took too big of a jump.
Trek Remedy: This was a fantastic bike, but what do you expect at this price point? Climbed well (though with 26 inch wheels and such a slack head tube, it wallowed a little bit. I could see how this could be a liability on steep ascents), descended well, and was a great all around bike. If I had the cash, I'd throw down $6,000 on one of these tomorrow.
Trek Stache: This thing road a lot like the Rumblefish save for two important differences. First, the steering was a tiny bit quicker at higher speeds in a very important way (you wouldn't get caught in tight switchbacks), and it wasn't nearly as plush (no crap, right?). It rode like a hard tail, but as a very playful hard tail. It could eat up bumps and crap in the trail with ease, but you still had to pick an actual line and get up off the saddle BEFORE you hit any rocks or roots (or said simply, it is less forgiving of sloppy riding). Overall I liked the bike. It's got a 90mm stem, which I THINK would be better with a 50mm stem as you said, but I'd want to ride it for a while to see if that's really the case.
Specialized: I hated this bike. I hated the people at the Specialized tent. They were all very mean, egotistical people that I want nothing to do with. I've got a Specialized Allez as my road bike, and I considered going home and selling it because these guys were such bags. Oh and the bike rode like crap. Who would pay $10,000 for this POS?
So after the demo day, my thoughts are still pretty much the same; Grab a Stache now and have a bike that I can do MOST trails that I want to ride, be able to race, and be able to travel with, or wait a few months and grab a heavy all mountain bike. This would either be a Rumblefish, a Remedy, or a Heckler if I can find one to demo.
If I get the Stache, eventually my stable will have the Stache (probably as a single speeder in a few years once I trash the drive train, though that's only a guess), something in the 5-6 in travel range (maybe a Blur LT2? Maybe a Nomad? Maybe a Tracer? Maybe a Bronson? Maybe a Tracer 275? Maybe a Yeti 575? Heck if I know, but you get the idea), and then a road bike. If I find that one way I want to go and jump into the park (quite literally), then maybe something along the lines of a Kona Operator?
Long post I know, but does that give you an idea of where I'm coming from, where I'm at, and where I'm going?
In the end, as I said earlier, I don't think there's a right answer here, and I don't think any pathway would be a bad way to go. At this point, what I think I'm going to do is go grab a Stache 7 and have some fun with it for the next few years, and see what I want to do with it when I move back to Utah. I can still travel with this bike and ride what I want to ride out west, and it'll give a chance to grow as a rider. I'd get the Stache 8, but frankly, I don't think it's worth the money. I've always run Rock Shox suspension before, and once you hit their air shocks, I've found they do a fine job (I'm a huge Fox fan when you get into higher end bikes on full suspension designs, but that's not what this bike is). The wheelsets are the same (though the 8 has a slightly lighter hub), I like Avid brakes well enough (as long as they're not Hayes. I hate Hayes brakes), and the drivetrains are similar enough. Long story short, I don't think the 8 is worth the extra money for me, for this kind of bike, for the type of riding I'll be doing. As you said, if I really want to lose some weight, it'll be in the wheelset; so I'll wait until I taco a rim and then grab something lighter and stronger.
So with all that...thoughts? Suggestions? Here's your chance to tell me I'm an idiot and I'll happily listen