Is this at 135? I see it as a pick em fight. Mayweather has a clear edge in boxing ability at any weight, but Shane was pretty dynamic at 135 and had a good workrate. Their fight in 2010 was entertaining because of the clinic Mayweather put on after the early scare, but i think a prime match between the two would be entertaining by how competitive it would be. If you put a gun to my head, I'd say Shane takes it in a close one. Still though, I don't think it's inconceivable that the fight plays out similarly to their 2010 match with Shane just bouncing around more.
Man Shane at 135 was a beast and might be too fast and powerful for Mayweather at that weight. Any other weight I pick Mayweather.