I think ALL of you guys are thinking to black and white.
It all dependz on weight class, and who was coming to who, now you say natural weight, Floyd has been known to walk around 170 so his natural weight could be close to middleweight.
At Middleweight Roy Jones would KO Mayweather in a matter of rounds, he would be too much for Mayweather as far as size and strength goes.
At WW Mayweather would UD Roy Jones easily.
You really can't say who would win, simply because neither has been to each others weight class, Roy has always been middleweight and Mayweather has always been WW.
Best way to equally decide this, if say, Roy had always been a HW and comes down to MW and Mayweather goes up to MW. 1 moved up 1 solid weight class, and 1 moved down 1 solid weight class, none of those fake weight classes.