I've beat the internet pretty hard and can't find much information on
air leakage by brand, let alone window type. According to the NFRC
site, the air leakage rating is currently optional on their stickers.

It is interesting, though, that Marvin double hungs and casements, for
example, have the same R-factor (2.7) and the sliders have a lower
R-factor (2.33). There could be things other than air infiltration at
work, of course.