I will be able to tell you more once they are up and running for a while, but I just set up a pair of these.
First of all, these are very heavy-duty units. I recently read about someone breaking off the outflow on their FX5 - that wouldn't happen very easily with these, as they are cast out of very thick PVC.
The stainless ring that seals the top is also a very strong design, overall they are very well built. This seems to be the only real difference between the Nu-Clear and Ocean Clear filters, otherwise they look to be an almost identical design. I've read that the Nu-Clear is easier to open/close, but I have never owned an Ocean Clear.
What really sold me on the Nu-Clear canisters is the 522's cartridge. It is an 18 square foot 100 micron mesh filter. Since particles don't get trapped in the fibers (so they say) you can just hose it out to clean, no soaking in bleach. By following this with a 30 square foot 25 micron unit I am hoping to optimize my filtration versus service time. That's my theory, anyhow.
I forgot to mention that I am using mine for mechanical filtration only. There is an area in each filter that could be filled with biological or chemical media, but I'm not using any.
As I said, I'll update in a few days once I have a chance to hook up my plumbing.
Attached is a photo of my Nu-Clear canisters as I have set them up. Iwaki MD40RLT - Nu-Clear 522 - Nu-Clear 533