Intuitively, I'd put my money on the 40/40 tank.

EDIT:
Oh, somehow I read the two choices as 75/75 and 40/40. Doh!
Yeah, no question, the 75/40 is faster than the 40/40.
/EDIT

Need to google around to discover how many BTUs are required to heat a unit of water by a given degree change though.