xp3 is underfiltered for a 150 gal tank IMHO. If your sucking sand with an xp3, your gonna suck a ton more with an fx5.
lets see, I have 2 fx5's, 2 x 404's (same as 405's), an xp4, a 40 gallon sump, a via aqua something or other, an eheim 2028, 3 magnum 350's with bio-wheels, and a few scattered HOT 250's. Not to mention several Mark V and Mark IV sponges. Did I mention I can't stand HOB's?
So, lets see, I like my rena, love my fx5's, think ehiems are incredibly overated, any fluval filter less than an fx5 is marginal at best (don't know about the new gx series) and love my magnums (although the bio-wheels I am not too crazy about).
As much as I love the fx5's, I don't use them on a discus tank, a lot of current, which can be overcome at the expense of throttling down the filter.
Honestly, I think the best filtration for discus is sponges with a magnum 350 or two thrown in for mech. Remember, just because the debirs is sucked in your filter and trapped, it is still very much in your tank. It only gets removed when you clean your filter. It's easy to pull a micron filter out of the magnum and replace it (an they can be re-used over and over, just need a few for rotation).