When shopping for sway bars, here are some things to consider:
1. Do you care about the weight of the bars? If you do, buy hollow bars since they're lighter than solid bars.
2. Look for weak spots - welded spots are typically weak points.
3. Adjustable sway bars are really nice, but you'll pay a little more for them. Even though the settings are usually limited to 2 or 3 per bar, it's fun to play around with them and it's really nice to dial in exactly what you like.
4. What's the intended driving purpose of the car: street or track? If that car is tracked, what are the class limitations? If there are none, get the biggest bars you can find. If it's your daily driver, you probably want to keep some understeer characteristics that the car comes with from the factory (typically, that means having a slightly larger front than rear bar). This means that at the limit going around a curve, the front end will lose traction before the rear. This is safer because as you slow down, the nose of the car will eventually regain traction and you'll be able to steer in the direction you want to, although the steering radius will be significantly longer and you may plow head-on into something that you didn't want to - but in a crash situation, this is actually safer than hitting the object sideways. If your car had oversteer - or in an emergency situation snap oversteer - the back end of the car would whip around at the limit or while swerving abruptly, causing you to lose control completely (unless your car has a ridiculous amount of power and you could mash the gas pedal to the floor and regain control with a drift - which very few of us have the ability to do).
...a good rule of thumb is to upgrade both bars at the same time and in relatively similar increments. You don't want to get only a huge front or only a huge rear bar because that would create unusual handling characteristics. You should get a pair of medium or large bars and install them at the same time (ideally, with upgraded endlinks too) for the best results.
I don't know how much you've read about this stuff, but that's most of what I've gathered. Aftermarket sway bars and endlinks were by far one of my favorite modifications. Good luck and keep us posted.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)