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The two filters have roughly the same size "media compartment" but since the bio media trickle chamber sits on top of the C4, the C4 actually has more total cubic inches devoted to mechanical and chemical media than the AC70.
The AquaClears flow from bottom to top while the Fluval C's flow from back to front. This means the velocity of the water in the AC's is nearly double the velocity in the Fluval C's. As a result:
1) there are holes in the sides of the media basket when causes much of the water to bypass the mechanical sponge and
2) you cannot use 50 micron polishing pads like to you can the Fluval C's. Thus, although you can get crystal clear and polished water for the "fish swimming in air" look by using tightly packed floss, it will never match the capability of the C4.
I use my AC50 as a "spare auxiliary filter" stuffed with floss & biomedia. I had to duct tape the side holes to make sure all the water passes through the filtere media.
The C4 media is very flexible If you chose not to use factory pads. You can use cut to fit media that makes the C4 very flexible. I have experimented with many kinds including AquaticLife non-woven polishing pads which are only 1/12 inch thick. Most recently I have been using in my mechanical chamber: Acurel debris reducing pad -->AquaticLife 300 micron -->AquaticLife 100 micron --> AquaticLife 50 Micron as shown here:
Dirty Acurel Pad + 300 & 100 Micron AquatLife Pads Being Replaced With New Acurel Pad + 300, 100 & 50 Micron Pads. 3-19-14 Photo by Paul1792 | Photobucket
Because there is no bypass issue, my water goes way beyond crystal clear and polished ... to the point of being "sparkly" & stunning ... as if I was running a canister and a UV sterilizer.
As for the huge chemical chamber, I stuff it with two 100 ml bags of Purigen (change each one annually at staggered 6 month intervals) a bag of Seachem Matrix, and a bunch of floss:
C4 Chemical Basket-2 Purigen, Matrix In Biobag, + Floss Photo by Paul1792 | Photobucket
I admit, if you are not as obsessed with making your water clarity a "showpiece" ..... the AquaClear will keep you fish healthy and your water crystal clear.
There is no question the AquaClear is cheaper to operate and I do like its re-usable sponge. To keep the water crystal clear, it probably only costs about $5 for a year's supply of floss (SeraWool is best) yet C4 factory inserts are over $2 a piece so monthly changes would be $24 a year. Even the off brand ones are $1.50 each on Amazon. Even my four layer "custom media" mentioned above costs about $1.50 per application
Other notes:
Fluval C's are much easier to service than AquaClears since you have to pull the entire media basket out (what a mess). The C's can be serviced while they are running and each compartment pulls out separately. I cut my custom media way in advance annually ... but with that out of the way or with the factory inserts, you can replace the mechanical media in one minute, I only have to pull out the chemical chamber twice a year, and the bio-screen over the drip tray takes about one minute every two or three months to squeeze and rinse out.
C series poly foam inserts filter down to somewhere between about 100-200 microns. I have inserted 50 micron pads in front as a last stage of filtration and get quite a bit of "gunk" captured to achieve optimal filtration.
