I don't know why, exactly, but Tristan seems a more engaging character than Melody. Can't quite put a finger on it, but such is how it seems.
Well, in a sense, Tristan has had a bit more opportunity to DO anything in the comic; we've only actually seen Melody in the cemetery with Erik, and she's mainly served as emotional support for him so far. We've really only seen Mel in the context of her relationship to Erik, and she seems to be going with the flow, more of an observer than an active participant in the story.
Tristan, however, came into the storyline mainly on her own, and has been questioning and second-guessing things from the start. We've also had a chance to get inside her head, something we haven't been able to with Mel. I think she seems more engaging because Mel has been somewhat in the background, while Tristan's been dominating the spotlight since she first appeared.
Any of that make sense?