I think GSP has already stated that if he is to move up to MW, it would be a permanent move. If he beats Shields (and I'm pretty confident he will, but anything can happen), it'll probably take at least half a year, probably more for him to put on enough muscle to feel comfortable in that weight.
If we take a look at GSP with about 15lbs more muscle, its pretty obvious that with that time, even with roids, his cardio is going to take a big hit. He is going to put on a lot of weight, so he probably won't be as explosive either, even though he might be strong as a bull at that point. He will also give a nice reach advantage to every top MW he is to face, not only AS whose reach advantage is going to be pretty massive.
On the other hand, AS is 35 years old now. Its pretty likely that since he has pretty much done all there is to do in the MW, he won't probably improve from here. His age is going to be a factor sooner or later. Very much like all the great strikers, one of his greatest advantages is his reaction time, which does slow down as you age. Your conditioning and your strength takes a hit, and cutting weight becomes harder. Not saying his time has come, but you never know when it will.
AS's weak point has proven to be wrestling. Sonnen ragdolled him around until the events in the 5th round, rib injury or not. Other fighters have had success with taking Silva down. Can GSP do it? The fuck do I know, but I believe that he has the tools to do it if he can adjust to the weight class. From there on, its all GSP's game. Can he finish Silva? Unlikely, but I also doubt that Silva can finish GSP off his back.
GSP's trainingteam might very well be the best in the business, and if anyone can come up with the perfect plan for beating AS its Greg Jackson & co. GSP is very well rounded, and I wouldn't say he is going to get destroyed standing in the 1st round. Clearly Silva has an advantage there, but GSP and his crew probably know enough not to completely play Silva's game standing.
With all that being said, I have no idea who is going to take it. Moving up the weight is always a big move, especially when you put on a lot of mass on the process. We've seen fighters like AO do it with great success. Of course the situation with GSP is completely different, but who can really tell how he will perform with more mass? That being said, I believe AS will obviously be a favorite here, but how big, I can't tell. Might be 60-40, might be 85-15, only way we can tell is if GSP beats up Shields, so thats what I'm hoping for.
Besides, its all basically playing down right the way Dana wants it to. If GSP wins Shields, the puzzle is ready. GSP moves up the weight, vacates his title and there will be BJ - Fitch for the WW belt and Silva - GSP for the MW belt, both likely to be massive draws (well, atleast Silva - GSP) and bring the much needed shake to the divisions. Win or lose, Silva could very well move up to LHW after GSP fight, and look to take the title from Shogun/Bones/who ever happens to posses it at the time.
With LW and HW being under constant changes (you can't really name a fighter high above the rest in either of those divisions), we'll have some really interesting times comeing up in the UFC. Throw a bunch of solid prospects in the mix, and we've got ourselves a pretty superior organization. Take the sport to Japan and increase the ammount of events in the Europe, and very soon MMA could become a mainstream sport world wide. Lets all just hope it all plays out that way, coz I know I sure will.