There's a significant difference in the maximum flow rates vs pressure in those pipe sizes. There are some charts here if you're curious but as mas985 said; larger pipe will be better (to a point). Personally I'd go with the 3". The power of the pump will play a role, but it won't change the general rule that 3" trumps 2". It could be that the first builder is using a smaller pump than the second, or just as simple as the difference isn't a huge deal. On the other hand, he could just be less knowledgeable than the second builder.


If you read the information provided here, you'll see that to figure it all out precisely will take a lot of work and a calculator... on the other hand, I'm guessing a pro pool guy probably has some more general rules they follow to make life easier.
http://flexpvc.com/WaterFlowBasedOnPipeSize.shtml