this could go both ways
in his shop if something comes up like a bad sheave or a bad anchor point for a shroud it would be cheaper to fix than if it was on the boat. also he may charge less per hour as he is not up in the air doing work on it, also some things can be done faster on the ground.
now the bad side somethings can be done faster on the ground but the price may not change, ie he makes more per hour. also is he local or is he going to charge and arm and a leg to transport the mast? the boat is not that old depending on how its been used it may not need a full work over. or the boat could be very used ( charter ) and it could use to be pulled.
what is his quote to do it off the boat vrs on the boat? and did you get a second quote from any one else.
this is a decision you need to make based on what you think as you know the boat better than we do. if the boat has seen light use i fresh water is a lot different than heavy use in the bahamas.
also get someone else to look at it