I love the Udvar Hazy, mostly because I went to the Smithsonian one about a hundred times when I was a kid.
BUT
I chose the Smithsonian. It's larger, has more exhibits, doesn't have $12 parking (the Udvar Hazy has $12 parking - $0.50 each way if you take the shuttle from the airport). If it's a total bomb, which I assure you it won't be, you can always go wandering about the mall. Then you can catch the metro over to Gravelly Point and pull out cameras and spot for a while.
Despite the UH Centre's tower, it isn't very good for spotting and the parking lot is actually the best spot for it. As for the exhibits, there are a lot less than the Smith, but that's also because they're still trying to fill it. The UHC has a couple more interesting aircraft than the Smith (Concorde, SR-71, Virgin Global Flyer, Space Shuttle, etc), but you'll be able to spend more time at the Smith (even in the A&S Museum alone).
So I recommend going to the Smith first and then the UH. Both will be around later if you have to visit the other on a second trip.
Either way, whenever you head to the Hazy, send me a PM, I can give you a ride up there at the very least. Afterwards we can hit up a Mexican joint that rivals or even out-does the mexican joints down here in Arizona.