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.
When I looked I saw that they had a deal for $95 for two years, but I never know what I'm doing that far in advance. Edit: I was going to cancel my auto renewal and just buy the cards. Amazon has them for 44.