I am no expert, but have always preferred canister filters with a large volume of media and not excessive flow-rate vs ones that hold less media but with higher flow-rate. Someone over here (Italy) who has more specific knowledge reckons that a flow-rate through the media in excess of 15 mm/sec actually becomes counterproductive as the reduced contact time of ammonia/nitrates with the biomedia reduces effectiveness. Now you will ask how to work out the flowrate INSIDE the canister? Somehow it is the ratio between the pump flowrate and the surface area of the baskets in the canister but it would cost me a fair bit of headscratching to work out exactly how to work out the math!