Watcher, i forgot to reply to your question. for each type of fitler i am mainly talking about whispers (and a little about emperor/penguin) and fluvals because these are the ones i have a lot of experience with.
canisters are better because:
-much more media (more filtration at the same gph)
-may be quieter (although i have bever had problems with my whispers, i sleep within about 5 feet of 4 whisper 60s, and when i was sleeping within feet of my fluval, i heard it and not the whispers).
-higher quality water, for things like sensitive species, or species with very specific water parameters.
HOBs are better because:
-easier to maintain (30 seconds as opposed to an hour or more)
-no more expensive than a canister to maintain if you buy cartridges in the 12 packs
-better water flow, it just seems that there are more low current spots in a tank with a canister than one with a HOB
-biological filtration probably not any worse than a canister's at worst. the bio media in a canister is in the airtight canister, so only oxygen dissolced in the water will get to the good bacteria. in a HOB, the water is going threough turbulence at the surface before it gets to the good bacteria, but the bio media is smaller on the HOB
-they are much cheaper. why spend many times what you need to on filtration that isnt needed in the first place
-most notably mechanical filtration. i have now had water turtles for about 8 years. i have found whispers to be the best filter for these messy animals. on reptile forums i frequently see people recommending canisters for water turtles. these same people are recommending very frequent water changes and tank cleanings. i had a whisper 5 (it took 2 XL biobag, they dont make it anymore) and used that for my 40 breeder. that tank had 4 4-5" goldfish, and 3 turtles averaging 5" each (one being a snapper, so he actually produces more waste than a basking species of the same length) before that filter could not handle the tank by itself. if a whisper can keep that tank crystal clear, with better results and less needed water maintenance than a canister, it is more than enough for even the messiest of fish. water turtles are many times dirtier than a cichlid. i have used whispers for all my 10 years of dealing with fish. they have always been more than enough. as said, unless you have very sensitive fish that need the massive customizable filtration media found in a canister, you really just need a whisper.