I adore
Marielle. It's a French diminutive of
Marie so Mirielle doesn't make sense to me. (OTOH, the lovely French
Mireille does.)
Marielle is not a common name so I don't think you need to get kr8eef with the spelling. I also think people would automatically assume the
Marielle spelling or either
Mariel (after
Mariel Hemingway). I don't think it's hard to pronounce at all, but I do think Mirielle looks contrived.