Rb- I modify the media in virtually every filter I own, and this includes canisters. Most filters come designed to do a bit of everythig- bio, mech and chemical. However, my experience it that most tanks usually want a media mix specifically for that tank. For me the appeal in the AquaClears is their versatility. I think they allow for a much greater assortment of media than most other brands. I also prefer doing spot Chemical filtration using a power filter- its a lot easier to throw a bag of carbon into one than having to deal with a canister.
The big difference for me between canisters and power filters is ease of cleaning. I just have to pull the frame with the sponges and floss to a bucket to rinse and replace. With canister I have to disconnect the hoses, bring the canister into the bathroom or kitchen to clean. Of course I have to do this less often than I clean the power filters. Afterage time to clean one AquaClear is 5 minutes of less while for my Eheims its more like 45 minutes.
But what I can say, is that one's choice of filter should be based on a number of factors. Trying to select the best filter for a given tank and application as well as appearance considerations dictates the choice in the end. I find it is much easier to hide canister intakes and returns than to mask power filters. Canisters also tend to have more media capacity.
For me, once you know the better brands to buy, the selection of a filter or combination of filters should be dictated by the specific needs of any given tank. I even have a tank which uses an Eheim canister, an AquaClear power filter and a hang-on-the-tank (H.O.T.) Magnum loaded 100% for mech waste removal.