Both are different but so off the wall that they will be misspelled or mispronounced. If you are interested in nicknames for her Matilda has more options (Matty, Tilda). I think Matilda Rose would be pretty too although you said MRS was a no go. I don’t think MRS is a bad set of initials though. My own (married) initials are MS. I knew a girl named Mary-Rose in high school as well and she didn’t have much trouble after the initial correction with people correctly saying both names.
DD1: June '16 DD2: March ‘19 :::: Married since 2011 :::: USN Wife ::::
I honestly don't understand the issue with initials. What's wrong with MRS if you like the names? How often do you actually use your initials. Not often at all. Sometimes I think we look too deep when naming kids.
Re: Stuck between two names
Of your two front runners I prefer Matilda Mary. Zelda is a little out there for me.