As a person who has built things too small before I would opt for the added two feet. I do not own a darkroom but have worked in ones that were too small and one that was spacious. The spacious one was about 10x12 and it was very, very pleasant to work in. I did 16x20's on a regular basis and the space was really nice. Trust me you will fill the space you build.
Mark's first law of space. If it is there it will be used.