They have a LOT more room for filter media, the media itself is customizable to what you want. They hold a lot more water too which is good if power ever goes out, a bio wheel would be the worst thing you could have because it will dry out and bacteria will die within an hour or two if you are not home to trow it in the tank. Canister sits out of sight and you barely see it in the tank either, instead of the giant HOB hanging on your tank. Canister is almost silent if not completely. Canisters have the ability to aim the output to where you want, generally in this sense they circulate the water better unlike the HOB on a long tank that just mixes a lot of the water on one end. Its mainly do to the way the HOB pours the water back in from above, where a canister can use a spray bar and shoot filtered water along the length of the entire tank. Canisters being as large as they are don't need to be cleaned as often. No expensive filter media if you wish to just use pot scrubbers and filter floss. Any filter media will fit in a canister. Canister does not disturb the surface making it more plant friendly then the HOB filter, which is the killer of CO2.
All this said I really only suggest canister on 40gallon or larger tanks. I know they make them smaller then that, but IMO they get too expensive. Like every filter though, don't go by what manufacturer says the filter will do. Go by what you want the filter to do. Turnover rate is important. A lot the "for Aquariums up to XXXgallons" do not make sense to me. Lets take RENA canisters for example since I am familiar with them. The XP1 does 250 GPH(gal per hour) and is for up to 45gallon tanks. If you do the math this filter will turnover a 45 gallon tank 5.5 times every hour. The XP3 though does 350 gph and is for up to 175 gallon tank. Doing the same math here a 175 gallon would only be turned over 2 times. I can tell you straight out that a XP3 won't do a 175 gallon all by itself. You may not need another filter, but you will definitely need a powerhead or two depending on setup.
I have had one HOB and that was a Aquaclear. It ran for all of two weeks before I could not stand the god awful noise it made. It ended up in an auction within a month of me buying it. HOBs IMO are very flawed filters. I am not surprised either that canisters are in the lead
. Again give me anything but a HOB.