8-3 UF wire si good for 40A.
Wondering why your electrician is suggesting 6-3? (which is good for 55A). Maybe because he already has some on had?
Also, not sure what type of breakers you have, but it may be a bit difficult putting 6ga on a 40A breaker.
As far as length, you can go to 125' pulling 40A on 8ga before you exceed 3% voltage drop. So I dont see that being a reason to upsize wire either.
Now you may want to see if there is anything you can do to get the circuit length to 50'. Wire comes standard in 25', 50', 125', 250, etc rolls. See if you can make 50' work and not have to buy 125' of wire, and not use 70' of it.
BTW, 50' 8-3UF is ~$90
.......125' 8-3UF is ~$190