My first answer was going to be that Marimite is superior in every respect (other than fan service); most importantly the writing. But then that's never guaranteed commercial success for anything, so perhaps there's something else.
There's a certain... honesty and lack of cynicism that Marimite has. A sense of innocence maybe. Sentimentality without being mawkish. It's hard to verbalise, but there's something about Marimite that produces bucket-loads of warm fuzzies like nothing else I've watched. In short, it's utterly charming. I'm guessing lots of other people feel the same way.
I'm enjoying Strawberry Panic, but it's just not in the same class as Marimite.
That's just my perception though. I'm a latecomer to anime and probably a fair bit older than most Marimite fans, so perhaps that affects the way I see it.
