There is not a huge delta in pricing. DH are slightly more expensive but they are more functional for most people (cleaning and ventilation).

If you get a Double hung that is airtight, the single hung will not provide much of an increase in performance at all.

+1 on WoW's comments.

I would also add that if the glass would break on the top sash of a double hung, it is much easier to replace than the fixed sash on a single hung

Also as an added note you did not mention if you were planning in using Wood or Vinyl. In wood units single hungs are actually a slight bit higher in price than a double hung because they are modifying a double hung to make it a single hung where as vinyl units are approx 10 - 15% cheaper.

Thanks for the replies.

I don't think I'd use the top ventilation if I have wood or woven blinds hung on them. Cleaning, though is a thought, especially on the top floor. We go to pick windows on Thursday.

Thanks.