If you think you'll get some great images with it, take the FA31. There's really no sense in having such a nice lens if you're not going to get every ounce of use from it that you want. Yes, there might be some risk, but equipment is to be used and enjoyed. I'm careful with my gear and look after it very well, but not to the point of leaving it at home.
By all means take whatever precautions you can, but use the lens... and, if you should be unfortunate enough to get condensation in it or damage it, accept it as "one of those things". You'll have some beautiful images, and the lens can be serviced. No big deal