My opinion is, and always has been that I will take Anderson Silva all day long in a 5 round fight and would start leaning more towards GSP in a 3 round affair.
The obvious has already been stated ten trillion times, GSP, while extremely dominant can't seem to finish a guy to save his life. In a 5 round fight against the single most dynamic and possibly feared striker in the entire sport I don't like GSP's chances of avoiding getting tagged for 5 rounds. Silva has two things that GSP lacks (or seems to) Killer instinct, and legit, no questions asked finishing abilities. So at the very least Silva would have 5 opportunities to put this fight to bed because GSP honestly lacks the ability to put away Anderson. This is assuming that GSP's wrestling skills will translate at a higher weight class because GSP relies on timing and speed to execute most of his takedowns and Anderson has some of the best timing in the sport and does not lack speed by any means. We KNOW how Anderson fights at 185, we can only speculate how GSP will, that tells me alot.
As I said though, in a 3 round fight, I like GSP's chances alot better. He has the steller wrestling to ride out a decision. Paired with his great game planning and ability to execute gameplans he has to tools to steal 2 out of 3 rounds no doubt especially with Silva's average TDD. Really it depends on what GSP looks and feels like at 185. I still favor Anderson though if this does go down.
And just to address the Sonnen/GSP comparison. And this is just my opinion obviously but there is a big difference between the two. Sonnen was able to clip Anderson and put him down but that is because Sonnen for a lack of better description is a crazy SOB and was not afraid of Anderson. And if my memory serves me, I never remember seeing Sonnen afraid to take strikes. GSP (and this is opinion, I'm not stating it as fact) seems to lose his composure when he gets punched in the face. He gets visibly frustrated and goes into survival mode. I don't like that against a guy who hits hard, accurate and often. If Silva starts landing on GSP and GSP can't get the takedown immediately he is in deep trouble in my opinon. GSP is a better striker than Sonnen but he does not like to get hit, again just my perspective.
Sorry about the novel. Bored at work