Check out that article. It's from last year and it's centered around games, but it goes into more depth on the different options from Nvidia and ATI. I found it good reading on the issue.
i kind of prefer nvidia's nview, cause IMO its a lot easier to fiddle with and all, in terms of the settings and preferences etc. but then i have friends who swear by hydravision and how much it kicks nviews ass. if possible, you should try both out, with friends' computers, and see which one tickles your fancy.