Speaking from my own experiences, here is a scenario you might want to consider:
1. I have enlarged to 12"x16" from FP4+ developed in Pyrocat. It is the only film I've been able to enlarge that much from 35mm while maintaining really good sharpness. I use Pentax SLRs and Pentax prime lenses along with Vivitar Series I.
2. A friend of mine uses a Leica with a 50mm Summicron, and he asked me to print a shot with Plus-X I could hardly enlarge beyond 9"x12" before it started falling off severely in sharpness. Developed in Xtol by him.
Same paper, same chemistry, same enlarger, same lens. I think this tells us that FP4 can be sharper than Plus-X, and I'm sure it can work that way the other way around too, depending on how the film was developed, printed, etc. I think that both films inherently have quite a bit of contrast. The grain looks really nice. In Rodinal, you may see a slight speed increase with Plus-X over FP4, but that's all I can think of that really distinguishes them as one better than the other. I will happily shoot either of these two brilliant films.
- Thomas