or you can go to http://www.rolandus.com and you'll find link to boss there.
And you won't find anything special in these 2 sites anyway, I believe http://www.musiciansfriend.com has more info on these products.
I recomend that you try both before you buy if you could. Also don't be so amazed by effects like auto-yaya, auto-riff and other useless stuff you'll find in the GT-6 or GNX-3.
For me I'd compare between the overall sound quality of the effects you know you'll use like the distortion types, the wah-pedal (although I know all multi-effects sux with the wah), delay & reverb ,chorus , compression & the Noise Gate.