FRA can be a bit of a nightmare at the best of times.

Every time the issue of FRA vs MUC comes up, it is said that MUC crews are better (no personal experience yet)

C seat is the same between the 744 and A343.

NRT takes a long time to reach if you're coming from central Tokyo, so you'll have to wake up ridiculously early to get the FRA flight which leaves at 0935, whereas the MUC departure at 1220 gives you some decent sleep.

Overall, I'd go for MUC.