Both. Or perhaps, something else.
My GF and I are into speedy K and MDA, respectively, so in order to sort out all our respective wants we need a Mandelin and an XTREME kit as well.
Of course, we'll both happily chow down on a good MDMA pill, so it's not like Marquis reagent isn't handy on its own.
It depends - if all you care about is getting MDMA, then just go the Marquis. If you are actively seeking other things, then carefully look over the eztest matrix of detected substances and do the math.
Confirming Ketamine isn't terribly useful if K isn't what you want.
[edit: removed confusing combination note]