That said, I also think it's very easy to move from a D or D- up to a D+ or C- (a winning season, regardless of rpi, will move you a 1/3 of a grade at that level, and a PI appearance will easily move you 2/3 of a grade).
So while I agree w. reinsel that, all else being equal, you should take the D+, I would say take a good look at the roster, location and conference as well. If the D or D- team has significant advantages in those areas and you feel pretty good about at least posting a winning record off the bat, that changes things quite a bit imo.
Some of the smaller conferences are quite active and populated, and others are ghost towns (really just varies by world). Personally, I wouldn't wanna be stuck in a conference with a bunch of sims.