@34AC - I found this thread very interesting. Yes QFX models are generally preferred option of similar EX models, although they both have a very common feature set. As has been stated, EX is more Enterprise/Campus focused, while QFX is more DC focused, although both can in certain instances fit well into the others space. From a test persepctive, EX is generally test with Enterprise/Campus use cases, while QFX for DC use cases.
I assume this is a new purchase. If yes, you should really be discussing this with your partner of choice, and/or local Juniper Sales account team. In general, there are lots more factors involved with such a purchase vs which switch is best for DC. I mean architecture is generally the first discussion point, and from there the pros/cons of one approach vs another, and one switch vs another can be vetted out.
I highly suggest you have such a discussion with someone local, who should then have a much better view of your current network and any new network requirements, both very short term, but also long term.
Of course as far as any answer is concerned, Juniper is the obvious best choice no matter what final direction you care to take.