Arthur, 36, has trained with both fighters and gained first-hand knowledge of their technique and training regimes – and the Scot is tipping Mayweather to add the WBO welterweight title to his WBC and WBA super belts when the pair meet in Las Vegas.
Arthur told Sky Sports News HQ: “I think Floyd is the better fighter. I don’t know about the tougher fighter but he’s the best. He’s a lot better than Manny Pacquiao and has a lot more weapons in his arsenal.
“They’re both very intense individuals and train unbelievably hard. Floyd trains for hours and hours at a time whereas Manny’s workouts are a little bit shorter but equally intense.
“Manny seems to split up his sessions more. Floyd seems to pack a lot more training into one workout.
“I’ve worked with both trainers in the past and they’re both are absolutely brilliant. I spent a month in the Top Rank gym with Wayne McCulloch and Roger Mayweather was there regularly and I had lots of conversations with him. I sparred his other boxers.
“Pacquiao has to set a fast frantic pace and throw a lot of punches. He has to keep Mayweather guessing all the time because Mayweather works out patterns and starts to judge him coming in and can take Pacquiao out of his stride.
“I think Mayweather has to maintain a work rate to keep Pacquiao at bay early. Whether he can or not remains to be seen because Manny works at a tremendous pace.
“Pacquiao will be looking to throw a lot of punches, find a lot of different angles and ask questions with things Mayweather hasn’t seen before.
“Pacquiao’s hands and feet work a little faster than his brain and he doesn’t think about what he’s going to do, relying on speed and volume. That could play into Mayweather’s hands.
“We saw how easily Floyd handled Marquez. It was like a father who can box playing with his son – it was that easy. Then you saw the trouble that Marquez gave Pacquiao. Those two know each other very well though, so you’ve got to take that into account.
“I’d have to go for Floyd Mayweather. All round he’s a more complete fighter and technically better. He’ll be unbelievably well prepared and the pressure is on. He’s got the undefeated record and if he wins this fight, for me he’ll be one of the top five greatest fighters that has ever lived.”