I would trade it in, especially for $35. You can always buy the game again when it hits the greatest hits list for $20.
I only keep the "true" classics from any console. While SPM may be a great game, the only true classic on the Wii (not counting MP3 yet because I haven't played it) is Twilight Princess.
For example, the only Xbox games I have left are the two Halos. The rest are gone. I haven't cleaned out my PS2 collection but it was never big to begin with, I loaned a lot of games from friends. In the case of PS2, there aren't many... GTAIII, Shadow of the Colossus, maybe a few others. GT would make it but I don't keep racing games, they age terribly.
For the GC, I kept RE4 and WindWaker. I was never a SSBM fan (not into fighting games) so that wasn't an issue.