I have to be honest here. I truly believe that this debate over canister filters is a little misguided. While I have done the "Great Filter Debate" myself several times, I think what it really boils down to is purely personal preference and reliability.
The Eheim is a tried-and-true workhorse that will probably last longer than the Rena. Both filters are good, but I would give the edge to Eheim in the reliability department.
Now. Back to your question. Almost all canisters are the same - media trays and a pump that forces water over/through these trays for your filtration needs. Surface area would be my #1 concern - biological filtration is what you need. Sure mechanical and chemical filtration are nice, but you do not "need" them in a tank. Assuming the Rena and the Eheim are closely matched in canister volume, I would personally pick the Eheim. The water is going to be in contact with the beneficial bacteria for a longer time, and with less pass-by, MORE of your water will be filtered as it passes through the filter. Essentially, the Eheim is more efficient. Hope this helps.
Last edited by ProndFarms; 11-21-2012 at 11:54 AM.
Reason: can't spell :)