[Peter Matteson]

There are several other threads where this has been discussed which you can read if you do a search but it basically boils down to several items already mentioned:
#25:
is strong enough for the drive train
is lighter
is more fickle about alignment

#35:
is heavier
more tolerant of misalignment
easier to work with.

My preference for drive is to use #35 and for mechanisms to use #25 in most cases. I don't like to have to worry about throwing a chain or even having to do real maintainance on the drivetrain which is what pushes me in this direction. Also I'm more comfortable with Al sprockets under that kind of wear at the #35 size. I know both work, but to me the trade-offs lean towards #35.