I know I'm not exactly the guest you may be seeking, but I do have an opinion for this.
I believe Marth dittos can accurately determine a superior Marth, but not right away, as in a few sets. I think that the match is constantly on edge during dittos, as moves can be punished in a deadly efficient fashion, or missed entirely, depending on the person's skill - so, I think that if one person is superior at either punishing accurately or fighting off defensively, that Marth will triumph more frequently over the other.
Say, I've had a roughly equal Marth friend for a long time, and we play a lot. Say, he keeps on baiting me to do things, and I fall for it, dying. Say, my defense game is so good, our matches are very slow, yet I beat him under the patience of my style. Regardless of one's style, aggressiveness, or simple "nuu, I get more tipz so im better" I think whichever Marth wins more often against their foe is superior, no matter how. If you believe otherwise, I will say that people are constantly improving, but I'm talking present, and perhaps always present. That's my opinion, though, that a series of clean victories (even if narrow) can make you a so-called superior to your Marth main foe, at least until your opponent decides to train their ass off for 69 hours and come back to destroy you, but again, present!
Also, if nothing I say makes sense, sorry!