I have both the QC20 and QC25, though I've only used the QC25's on two trips (one TATL, one domestic).
I haven't had any problems with perspiration on the QC25's -- I wore them for probably 5 hours at a time on each flight (on the TATL I took them off to sleep -- had a flat bed J seat
).
I still use the QC20's when I'm riding the bus or at the gym. I've never got a really good fit on them (tried all the end pieces) so I don't get the isolation that I do with the QC25's -- not such a problem on the bus or at the gym.
So I like the sound on the QC25's better and I find them comfortable to wear for hours at a time.