The overwhelming response I have gotten from people I have talked to that play Guitar Hero, from those starting out to seasoned people who can 5 star EVERYTHING (to whom I say, I hate you >.>) is that, while Guitar Hero World Tour was good, it's not as good as the other major releases (save GH).
I'd recommend either keeping GH3 as the game to use, or use Guitar Hero:Smash Hits, if it turns out to be pretty good. GH3 still has the toughest songs overall (Yeah, when someone can convince me that Satch Boogie is tougher than TTFaF, MAYBE I'll reconsider) and definitely showcases the series' merits. I'd personally weep if the final song of the GH tourney this year was The Devil Went Down To Georgia DLC on Expert.
As for Smash Hits? It threatens to unseat GH3 as the toughest game in the series due to its high amount of final set songs from the previous installments. Freebird/Through The Fire and Flames/Play With Me/Bark At The Moon/Raining Blood remade? If you REALLY want people to show off their skills, I'm thinking Smash Hits is going to be the best one. Not to mention that it's probably going to be the most sought after since it has many, many of the fan favorite songs.
World Tour added the new mechanics for guitar and that made it awesome. The whole band? No, Rock Band is still better. So World Tour is left competing directly with GH3 and, in my opinion, Smash Hits. It doesn't look good for WT against Smash Hits and I and MANY others still prefer to play GH3 over WT any day of the week.
Sure, there's still some time to think about it and I'd recommend NOT making ANY decision until after Smash Hits is released. But please, consider what I've said.