After thinking about your question for a bit. I can not think of a good reason to not have a 90% in a colder room.
I just recently assisted in the install of 3 units in the attic space of three homes. This space may be a little warmer that your basement but not by much that is for sure. All attics must be vented to the outside of the home for proper air flow during the hot season. So these heaters are subjected to some pretty cold temperatures during the winter.
As for which one is going to be cheaper the answer is simple. The 90 may cost more now but the overall savings in the future will pay that added expense back many times over.
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