GSP is Rashads former training partner, and they probably still roll around sometimes. It's easy to go with your former trainer partner when he's a champion.
The thing with GSP, and I think Matt Hughes put it best, is that he's not that great of a pure wrestler, but what makes him so good is that he's able to set up his take-downs with good combinations. Whereas a pure wrestler like Chael Sonnen can come in and take someone down right off the bat. If GSP can't set up combinations against Anderson Silva, it's going to be more difficult to get a take-down against Silva than it was for Chael. While I still think he will be able to take Anderson down, he's got to go 5 rounds without being put out cold against the best finisher in the business...