<p>You're correct about minutes-long auto exposures with a battery in the 3a. I can't recall just how long. A nice feature.<br>
I wouldn't question one camera's reliability - shutter or otherwise - more than the other, with one caveat - recent service. Whatever FM2n I might buy from an unknown source with an unknown history, I'd have it serviced immediately and then take it for much field-testing before going <em>remote</em>. If a 3a looked real good and performed flawlessly through several rolls, I'd skip the service.<br>
But which body? The newer of the two. I'd take advantage of advances and sophistication afforded by the 3a, incl Exposure Lock. If shooting much chrome, definitely the 3a as exposure adjustments are easier to discern on the needle meter vs LED meter of the 2n. Fill flash and flash comp are also easier on the 3a.<br>
Use 3v lithium cells, esp for frigid temps. No reason to ever run out of juice.<br>
Been shooting and beating many of these bodies for years (2 FM, 3 2n and 3 3a) - happy to have any for rough 'n tumble ventures.<br>
PS - If you're on a career-dependent assignment and it goes south, the let-down will be more than personal. 'Career-dependent' connotes mandatory success, back-up, etc.</p>