Considering the downloadable content comes promptly every week, Rock Band songs will eat a normal Xbox 360 hard drive within a matter of weeks. With Nintendo's stubborn refusal to add ANY hard drive for the Wii, it's unrealistic to expect the same level of downloadable content.
We can go on about Wii owners getting the shaft. As far as Guitar Hero 3 goes, no downloadable content was the least of their problems. It wasn't even in STEREO! I think Wii owners will just be happy to get anything at this point.
We have this. I've never played it but my wife has logged about 18,000 million hours at it. It is her #1 most favourite 360 game and my #1 most common reason for not having access to the TV. I guess that means it's good and you should probably get it.