Another Rena/API XP fan here. I have two 125's, each has an XP3 and an XP4. I load both of them with Eheim mech.... with much more in the XP4's. I've replaced an impeller on my oldest one, and that's it. I've also had a few Magnum 350's with BioWheels.... my XP3's are now running the biowheels great set ups!
As for Eheim, they really are top notch. Not sure why folks find them expensive. The Classics are pretty well priced. Yes they're a little slow on the GPH, but Eheim's ratings aren't "cooked". I have an Ecco canister that I got in a trade. Great little workhorse. Quitest filter I've ever seen.
With the SunSun's, don't cheap out on something that lives below the tank.
I'm surprised to see so much support for HoB's. I've owned a bunch, and they all drove me crazy over the years. The reason they're not "nitrate factories" is because they have so little media. I feed reasonable amounts, and rarely open up my canisters. Maybe once every 2-3 months. If I open them any sooner, there's little to find in them. The Eheim mech keeps them from clogging, and my nitrates are quite low and stay constant after weekly 50+% water changes.
-Ryan