Grips are more a personal preference thing than any other part on the bike. It depends on the size of your hads, how badly you want a certain color, whether or not you fix your bike yourself ( Only really relevant if you ain't running SS).
If you have gears, I would start looking at the new(er) ODI Ruffian MX grips. The half waffle on the back, and diamond on the front have proven really comfy for a thinner grip.
If your SS, I am having really good luck with my Fit Logo Grips, they haven't worn down much, but then again, since I got my big bike built up, the hard tail kinda jsut sits there more often.
The only reason I mentioned whether or not your have gears is this: If you are running SS, there really isn't much need to take the grips on and off much, so that means you can have any grip you want, because it won't be coming off much anyways, so if its not a lock on, you can glue it no issues. Chances are if your runnning gears, you are going to be taking the grips off much more frequently, and this is where lock-ons are PRIMO. My 2 cents.
Whatever you like. Completely personal preference. I like bmx grips for everything, other people like lock ons. After you have decided on lock on or bmx style, just go feel up some grips . Everybody likes a different pattern/softeness/width. Just go for what feels right on your hands.