The TRD one is right around $300 & I've read the most positive comments about that one as it has a lower inlet that attaches to the bottom side vent near the wheel. Of course, it's also been out longer than the AEM. I don't know the price of the AEM CAI yet... anyone?
Or, you could always build one yourself using a material besides metal for less heat soak.
Depends on how you drive... if you like to hear the sound & want the added acceleration, you'll use more gas, but if you drive conservatively, mileage could go up. It's my experience that it will go down. Why get it if you don't want to use it?