The gallon size was selected to normalize the discussion. The selection would be based on several criteria, not just number of units per volume - You are going to support a certain water volume in the tank, with the Coralife spheres you will need twice the packing volume = twice the filter size versus the Dupla spheres. If you don't mind going to larger canisters, this would be fine. But if you are going to be limited to larger spheres I'd probably opt for W/D rather than canister filtration or at least consider such.
Ehiems, bcause they are just made or superior materials to that of fluval, filstar, and (err) magnum. i have a fluval 304 on my 55g but i now regret that decision cause my friend has a eheim pro 2 on his 55g and he always has the clearest and least smelly water! my fluval is good but not as good as the ehiems. the fluval i have has a biowheel pro 60 so that i can have micron pads a carbon inside. it works but probably not as good as an ehiem.
i would say ehiem tho more expensive, but worth every penny!
im have a fluval 202 on a 3' tank, and a rena xp4 with an ehiem professional II 2028 on a 6' tank..
my favourite would have to be fluval just over the eheim because you get a nice separator for the media basket and the hosing for me is much easier to clean. (if only the eheim came with a seperator!!! ) both the eheim and fluval seem to be equal in features and quality apart from that.
quality wise the rena seems to be a bit less well made than the others, and it has no manual primer (on mine you have to fill the intake tube with a cup to get it going and its really needs you to soak the filter material every time) but otherwise it works fine.
ive only had these for about a year so i couldnt say for the long run, but for general features and price would recommend fluval anyday.