The mechanics have a little to do with this. In Rock Band the band as a whole can save a track even when one member falls down, and you can always use Overdrive (Rock Band's version of Guitar Hero's Star Power) to bring a bad player back into the song. In World Tour the failure of one player will nearly always result in the failure of all four, and while any player can use the Star Power that the band as a whole has built up to save their own bacon, the effect isn't quite the same. If you want to play with three mates in the same room, then Rock Band (and potentially its sequel) is still your game.
To counter that, World Tour has the stronger package when played online. Not only can you start a band online or hunt for an ad-hoc one, but the additional cooperative and competitive modes make for an enjoyable quick-and-dirty blast of online showboating (or humiliation). If you're looking for a game you can play on your tod, rather than a party game, this tips the balance back in World Tour's favour.
Of course, we still haven't got to World Tour's other major selling point: music creation. Effectively, you can use the game as a virtual studio, laying down tracks of guitar, bass, keyboard and vocals over either a preset drum track or one you create yourselves. The big problem, you might think, would be the use of a controller that only provides five notes, but World Tour gets around this by allowing you to switch between different scales. Making great sounding music isn't easy. Heck, making something even vaguely listenable is hard. However, it is just about possible, and the new GHTunes online service is already sharing user-created tracks that show some of World Tour's potential. Will anyone ever create a hit using World Tour? Probably not, but then I never thought GHIII prepared you for a six-night run at Wembley Arena. It's fun, and if it gets would-be Guitar Heroes a little closer to making real music, all the better.
And, in the end, fun is what Guitar Hero has always been about. I still think Rock Band gives you a better feel for life in a band, but World Tour is - even more than Guitar Hero III before it - the better game for those wanting straight single-player and online action. Of course, my opinion might change when I get my hands on Rock Band 2, and before you commit to either line of products you might want to check out the relevant setlists and DLC packs to see if there's anything that pushes you one way or the other. If not, then think about how and who you play with and - if you want a guitar game - World Tour is a good way to go.
Guitar Hero still can't match Rock Band for real band atmosphere, but World Tour is the better single-player game. What's more, the various tweaks and new features make this the strongest Guitar Hero yet.