Bro...You are everywhere.
I have both FX5 and Pro3. Here is my take on both:
FX5: Bigger flow, self priming pump plus some features (that I never use anyway). But most of all, around half the price of the Pro3. But as you mentioned, not much space for the media. Also, a lot noisier that the Pro3 and that matters to you. I find it a lot less elegant looking than the Pro3 also.
Pro3: Less flow, but still pretty good. 2 inlets which cover more area in the tank. Comes with the ''rainbar'' (if you like it). A lot of media space, can hold 8 liters of substrats and 4 liter of prefilter. You can get the media kit sold for the Pro3, it fills the canister really good. Nice sleek design, and really quiet. But costs a lot more than the FX5.
Here in Montreal I got myself the FX5 for 250$ and the Pro3 for 450$. But for that size of tank and a comm. of monster fishes, I would put at least 2 FX5, or maybe 1 FX5 and 1 Pro3. But people told me that a sump would do a better job. Maybe you should check into sump too.
Hope that helps.