I'd go with Masked Marth. At least they can be a free unit while Lucina is hard to obtain. Plus MM can be pretty much anything with a blank skill slate.
A +7 Masked Marth has the same attack stat as an unmerged +Atk Lucina, not to mention you'd have better chances of getting M.Marth to +7 before you can even get another Lucina.
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