Which bike would be a better climber and which bike would tend to wear you out quicker, one that is slightly undersized or oversized?
I think the size smaller would be a better climber. It may not wear you out quicker, though, being lighter.
Possibly, the size larger would wear you out quicker, as it's a bit heavier, more cumbersome to manuever, and you have to be vigilant about that top tube. The vigilance (OK, call it fear) of that top tube made me a better rider, but the one time I forgot, with slick cleats on a sudden hill.....well, it wasn't fun.