You'll find that anyone has done this will say that hands down, this is one of the most impressive mods you can make to your quad. Right up there with suspension, on my next quad, I would do bars and risers along with suspension before I did anything else period.
Solex tend to be the most popular but I'm not sure why other than b/c everyone else does them, and love them, most everyone else goes with the same. I'm running Solex myself but had ordered Rnethal fatbars first. Since the Rnethals arrived first, they got installed first. Once the Solex bars came in, I swapped everything out and just stuck with them rather than swapping again. That being said, I could tell no difference in the Renthal and Solex bars in comfort or vibration reduction as they were both identical. I went with a Oury grip which is very cushiony and love the setup. Folks that I know that have stock 660 and 700 bars sit on mine and can't believe the difference. On a long ride, it also severely reduces arm and wrist fatigue and cramping due to the different angle of the CR-HI bend of the aftermarket bars. CR-HI is the preferred bend for our quads by the way in case you didn't know.
I will warn you though that I've read on other forums that people with Grizzly 600's have had cable length issues if trying to go to a 5" riser and have had to use a 3.5" riser. On the 700's, the cable length isn't a problem when using a 5" riser. I personally have not done the mod on a 660 and tried a 5" so it may or may not be a proven problem. It just seems like there was a lot of talk on GC about the cables on a 660 bein' too short to use a 5" riser. Just keep that in mind.
Buster