3.08 or 3.73 Rearend

[72peace]

I've got a 69 with a 250 6cyl, manual steering, manual brakes, three on the tree and the current rearend is a 3.73. I've got a cleaned up 3.08 rear I was thinking about replacing it with for better gas mileage. But my mechanic say's I don't want to do that because the 3.08 will hurt if I try to tow anything. What do you guys think? I may do some minimal towing but nothing major. Not sure what to go with, help me out here!

Thanks guys,
Nathan