My experience is exclusively with the Fomapan 400; I've never used Fortepan 400. I'm using the Fomapan 400 as my standard ISO 400 film because I like the character of the grain; in my subjective opinion, it gives a nice moodiness to the photos, which fits well with the sorts of subjects I shoot. It's hard to describe better than this, though; this is a very subjective judgment.
Contrary to what I read everywhere, I don't find Fomapan 400 to be grainier or slower than most other "ISO 400" films, although as just noted, the
character of the grain is different. A few months ago I followed
this procedure and found that Fomapan 400 produces ISO 320-400 in PC-Glycol. (My camera's meter produced the same shutter speeds and apertures at both ISO 320 and 400, so I'm not sure which is the better speed for the film.) Of course, it could be that PC-Glycol is better at producing (at least close to) the rated film speed than most developers and that you'd get closer to ISO 200 in, say, D-76.