I've used both brands and believe that they have nearly identical sizes as far as filter media goes - it is likely that media from one brand may be able to be used in another.

That said, I prefer the Nu-Clear canisters because the retention mechanism used to seal the Nu-Clears (a screw-down pressure ring similiar to what I've seen in some pressure cookers) is much more reliable and leak-resistant than the twist on lids of the Ocean Clear canisters. The Ocean Clear canisters that I've used would not properly seal until I tapped them tight with a rubber mallet (as suggested by the Redsea help line that I called when my Ocean Clears continued to leak!). IMHO Nu-Clear has a superior seal and that, at least to me, makes all the difference.