If I were doing it today I have to regretfully say I'd probably do Airnet. From what I can tell they work a lot harder, generally at night vs. a lot of day runs at FLX, and of course it's a Big Company and I get the impression (from the outside) that there might be a little bit of Big Company BS that goes along with that. I've never been able to figure out the pay structure at Airnet, but at FLX it's not difficult to make high 30s your first year flying maybe 4 hours a day (but sitting at an FBO for another 6-10). Overall impression is pay is a little better on average at FLX to start, but airnet will pass it second or third year, plus you'll be in a lear. I could be wrong about all that.
On the other hand, well, what everyone else said. In a few years you could find yourself typed on a jet with 1000TPIC. That's pretty hard to look sideways at.
The only person I can think of who I'd recommend FLX to over Airnet is maybe a guy who already has a lot of time and is looking for a place to hang his hat and ride out the storm. It's a fun job and when you get a run you like, it's pretty easy too. Great management, zero BS.
Frankly, though, the way things are right now, if I were a low time guy I'd take whichever one called first and thank my lucky stars I'm not getting furloughed from an RJ.
Good luck.