Well it depends on what you're looking for. If you want a lot of action and humor go with SG1. But keep in mind a lot of the acting is bad, and the FX are horribly dated, way worse than TNG's. And half of the planets they gate to are just the same forest again and again. The character's are very likeable. It is more of a fun show without getting too deep. Although there are a lot of changes, this show is most similar to the original Stargate movie so you might want to check that out 1st.
I can't give you any good reason to watch SGA. That show is wretchedly awful and unwatchable to me. Awful characters, awful acting...just awful. It has the same tone and quality of an average Sci-fi original movie. Nuff said.
SGU is darker, has amazing FX and they actually spent the $$$ to hire some good actors. People criticize it for soap opera elements and being kind of slow. Since there has only been one season, it is a good chance to jump onboard with it. And it doesn't require viewing the previous shows to understand what is going on, which is a big plus to someone like me who wasn't a fan of the older series. If you couldn't tell, I'm a huge fan of SGU and not so much the other shows..
So make of that what you will, but I'd go with SGU.
Hmm. I emphatically disagree, the effects are definitely better than SG-1 and the acting is too, in my humble opinion. Also, I really enjoy SGA, not quite as much as SG-1 but it's still one of my favorite shows.
I'm actually enjoying SGU now that I've come to stop expecting it to be like SG-1 or SGA in any way, shape, or form. It's dark, which I don't mind, but I'm really enjoying that it's mostly character-driven conflict as opposed to the System Lord or Evil Wraith Plot of the week. Which I also enjoyed...I guess it's just nice to watch something different.
Oh, and about the same forest thing...O'Neill actually mocks that in one episode, saying something like "Oh, look, trees! Trees, trees, and more trees. Never seen that before." One of the things I like most about SG-1 is that it doesn't take itself too seriously, while SGU tends to take itself a little more seriously than it should.
So in conclusion, SG-1 and SGA are both great in my opinion, but you should watch at least part of SG-1 before you watch SGA. Also, if you're only interested in the first two shows because of SGU, you will probably be disappointed because they are really polar opposites. And SG-1 Seasons 9 and 10 are also pretty different but they have their moments.