I'd look into using a double casement. It will cost more but it's worth it. Horizonal sliders can have high air infiltration rates, plus they would be very difficult to clean unless they swing in.

+1

Square double hungs are goofy looking too!

I would go twin casement like Sky recommends.

Casement windows are the most energy efficient, I have heard. When the wind blows on them, it forces them to shut tighter, plus the lock snugs them up very tightly.

Thank you all very much. I'll ask contractor to price casements.

Casements will have lower air infiltration rates than double hungs (some more than others) and appreciably lower air infiltration rates than 2-Lite sliders.