You sound exactly like me 8 months ago when I got my new 125 gallon....I'm pretty sure I posted on here somewhere that i wanted the crispy water haha.
Only difference is I got my 125, cabinet, and 2260 modded with 950gph pump and extra double tap (making it a 2262) with mech and substratpro for $300 total.
Anyways I run the 2262 with 2 bags of Purigen and 2 layers of Poret foam, plus a ton of substratpro and mech. It'll keep my moderately stocked and probly overfed 125 CRYSTAL clear for a week and REALLY clear for pretty much as long as you want to go after that before changing the water (i've gone 4 months without changing water but still testing and all important readings being under 5ppm). If you can afford it, eheim 2260 or 2262 are the ultimate and super quiet. To get a better idea... my 2262 with 950gph pump is quieter than my magnum 350
I bought the magnum 350 with micron cartridges cuz thats what i read would get the crispiness...it does keep the tank clearer for a couple days longer than just running the 2262, but thats pretty obvious. Problem is you have to change the micron cartridge really often (5 days max on a big tank) or itll REALLY slow down the flow till it stops completely. To clean it you have to pull the whole filter apart, switch cartridge, leave micron in bleach overnight, get filter running/burp it, then 5 days later pull the whole thing apart again, put back in (this is assuming u bough an extra micron cartridge for another $8)...gets too annoying to be worth it. Plus you have to keep buying more of the sponge sleeves that go around the cartridge for like $5-$8 each, which become unusable after a couple months...they basically disintigrate. And you cant fit any other type of media in the 350 while it has a micron cartridge in it, which i guarantee marineland did on purpose. Really a water change is easier than using a 350/micron cartridges.
I've never had an Fx5 so I can't compare but I'll tell u the 2262 is amazing. Although $480 plus another $100-$200 for media is just way too much to be honest. Its a tough choice, but I dont think pay that much, at least as a recent college grad. Try to find a used one or something.