There are so many models out there, it's all in what you want to do with it. You can get lights, bipods, battery storage, pressure switch housings, folding ones, short ones, ect... I think if you heed to the old adage, "You get what you pay for" you'll be alright. Although some of those grips out there are WAY overpriced.
I have one of the folding/locking grips. They are ok. The locking mechanism does allow a slight amount of wobble though. I use it because it allows my AR to fit in my gun cabnet properly (when folded).