It's up to you.
RM has some really nice ships at most tiers up to BB4 level, including the excellent Capitani Romani, the insane Caio Duilio / Conte di Cavour, and one of the best EBB4 in the Project 41-B. However, once you reach BB5+, you have arguably the worst ship at every tier from then on.
MN has much less impressive early ships. Lower BBs especially tend to be either slow, lacking firepower, have low range, or have terrible spread even with high level gunners. Sometimes all of the above. A few are fun, but only with higher level crews. Even the PBB is a pain at level, though it does have arguably the best PCA. Once you hit BB4 though, you start getting fast ships with good speed and firepower (or at least tight spread to make up for lower gun count), including the Richelieu 1951, which is up there with its RM counterpart as being one of the most OP ships at BB4 level. And from BB5 on, you have the best aggressive ships in the game, though the apex is at the 6.5 (Bourgogne) level rather than the rather lackluster BB7.
If you're going for HA ships, MN is a no-brainer. Otherwise, it's the question of the better later ships vs the better earlier ones.