My sparring partner has a Marth although he's just one of his many pocket characters. I've yet to find the mu difficult although it is more troublesome than his Peach. I find no difficulty in the Peach mu, I'd just like to see what other M2 players have to say on the matter since it's easily my most played one.
...You can kinda say that about a large portion of the cast. Actually, just looked at Kurogane and only 6 characters are faster than M2 on the ground when dashing and only 2 are faster in the air and not a single character can claim to be faster than M2 in both departments. Walking increases the amount of characters faster than M2 on the ground (including Tink) and both walking and air acceleration need to be taken into account but point being M2 is faster than every character either on the ground or in the air, if not, both.
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As for Lucina and Marth, I'd be fine with them being put into a single thread. They share the same frame data and attributes (run, walk, air, fall speeds, weight, gravity) and nearly similar hurt and hitboxes (in terms of where they hit, not their power) with Lucina's being ever so slightly smaller in both cases. They have similar gameplans I'd imagine. Maybe not if Lucina doesn't out range us. The main difference to talk about would be if we can easily avoid Marth's tippers. If yes than Lucina is likely the bigger threat since she can kill us earlier than Marth can if he is stuck landing sourspots on us.