You know their frame data is the same right? xD So their agility is exactly the same.
Both Lucina and Marth are viable for their own reasons, but Marth is fundamentally better. So we make of that as we will, just don't assume that Marth vs Lucina is like a 100 - 0 MU or anything xD
Please refer to this for the actual property differences, and you can make of it as you wish:
http://smashboards.com/threads/before-you-ask-the-differences-between-marth-and-lucina.376180/
Marth, besides having to think more, has a lot going for him against most of the cast. However, fast characters can ruin his day, and against a semi ditto match like Lucina, will pretty much be evened out (though it is tilted in Marth's favor). Floaty and short characters can mess up some of his setups/combos, but otherwise aren't too bad to deal with. Heavy characters... let's just say Marth can do as he wishes like he had to them in the past ^^" Though that last remark can be taken with a grain of salt, since I'm yet to really look into the matter.
Anyways, I hope this answers your question (cus the last two times people have asked this very question, it's led to some pretty nasty drama that I'd prefer you be spared of ^^").