I can't comment on the 914 since I've never had one, but my 2002 was probably the most fun and direct car to drive that I've ever had. There is a lot you can do to the motor performance -wise and visibility is excellent (probably why some people said its like being in a fishbowl). Lots of room on the inside for being such a small car. Rust was really only a problem on the rear strut towers, but a/l/a you know what to look for--you will be okay. Just do your research and legwork no matter what you choose.
Maintenance is super easy on the BMW and probably has easier access too. Its very balanced and would make an excellent auto-cross or weekend fun car. Parts were fairly cheap and there is a huge following for them. I let the one I had go a year or so back, and regret it to this day.
I'll let you chew on this for a while:
BTW--this is Schwarz Black