I had stock grips for awhile.Then I switched to thicker ones for awile.Now I have the 100th Anniversary grips.They are thin and I like them just fine.I didn't see much difference in thick or thin.
I have had both and I prefer the thinner just becauseI can reach the levers easier. The Arlen Ness grips I have on mineat the present timeare of the thinner variety. I have usedthese for 3 seasons now and using anything different would just seem foreign.
Just like everything else, it's a personal choice. I really like the thiker grips. They feel much more comfortable, but they do make it a little tougher to reach the levers.
For me, bigger is better. Less hand fatigue, less squeezing. Still running the stock ones on my '99. I'll update to new big diameter sometime in the future too.
I have larger grips from the SAC Roadstore and they're really nice grips. All hand made, very cushy but kinda bulky. I don't know if they'd be good for anyone without really big hands, which is why I bought them. My finger tips easily touch my palm with the stock grips and my hands were cramping up bad.