Both the FM and the Nikkormat can use AI and Non-AI lenses. The FM requires stopdown mode for NAI’s and the Nikkormat requires stopdown mode for AIs that have no prong, like Series E and AF lenses.
Non-AI lenses are much cheaper than AI’s since they can’t be used on most DSLRs.
If you want to use lenses on a DSLR also, get an FM. If you don’t care about the digital option and want lots of cheap, high quality lenses, get a Nikkormat.
I got my FT2 (modern battery) on ebay with a great NAI 50mm F1.4 for $50. An AI 50mm f1.4 costs many times that because of the demand from digital users.
I used it recently for this portrait of Judith Berkson, which was printed in the New York Times last week:
http://www.nytimes.com/2010/05/30/arts/music/30playlist.html?scp=1&sq=judith berkson&st=cse