just with respect to these two cameras, if you want a simple fuss-free solution, i would suggest f40 fd as it probably handles noise better.However, i'm not sure how much the lack of IS will affect you.
reasons why you might want to get the f50 fd, it allows you a wee bit more room for creativity with shutter and aperture priority. Not forgetting that it has dual IS and also a long exposure mode that allows 8 seconds shutter speed as compared to 3 seconds on the f40fd.
So it's basically noise+features+more details vs less noise, that's the way i see it. so far shooting at low isos, i find that they're still manageable but i've sort of made a personal oath to not shoot at 800 or above. haven't tested 400 properly but 100 and 200 do seem acceptable to me personally.
btw, since you brought up only these 2 models, i assume you do know that you are limiting yourself to lithium ion batteries. and if you really have infrequent access to powerpoints, you'll just have to invest in more batteries.