G21C,
I've owned a number of 4x4 ATVs, both with front lockers, limited slip differentials (Honda and Can Am), and Polaris AWD. It depends on what type of terrain you are riding on, but for the terrain I have to ride on frequently I think the Can Am visco lock system works very well. I even prefer it to a full locker. The terrain I am talking about is like in this picture:
The ground is pretty much covered with ball bearings, and it can be pretty tractionless if you have to climb something really steep. The visco lock seems to let the ATV roll right across the top of the ball bearings without bouncing, bucking, and chucking the rocks out from underneath it like full lockers do. Also, full lockers are open diffs when not engaged, so they don't work well when they are unengaged, but are harder to steer when they are engaged. For this type of terrain I think both the Polaris AWD system and the Can Am visco lock system work better than a full locker. And they all work better than the limited slip in a Honda
2010 Can-Am Renegade 800R X Xc
2009 Can-Am Renegade 800R
2009 Arctic Cat Thundercat 1000
2009 KTM 505 XC-F
2009 Yamaha FZ-1