I think the smart money is on Mayweather by decision or a late stoppage. McGregor is a tough as nails but his cardio will be called into question if he cannot land on Floyd and Floyd is lighting him up at will. Boxing cardio and mma cardio are different. Even little things, like wearing boots, and bigger gloves will play a factor in a taxing fight where you aren't used to being in that situation. It can also be mentally taxing against someone as seasoned as Mayweather.

McGregor's best chance is an early KO or for Mayweather to have age incredibly since his last fight two years ago. Alternatively if there is a very lenient referee that let's McGregor get away with mauling Mayweather. That is probably the X-factor. The degree to which McGregor can implement an mma style game in a boxing match. If he can implement some 'dirty' mma tactics (particularly in the clinch) it may make for an interesting night.