Well, here are my two bits... I was considering the same sort of thing a month or so ago... I am glad I chose a canister myself... But both are good!
Would it be better to buy a canister filter instead of a hang-on?
"better" is definitely a matter of opinion in this case... They do have larger capacity for filter media, meaning more filtering power if that's what you are asking... However, HOB filters are very cheap...
Are they noisy?
My two canisters (Eheim) are very quite (in fact I am sitting right by my aquarium right now and I can't hear a thing)
Difficult to maintain?
Not really, you do have to worry about them clogging from time to time, but depending on your media choice they don't require a lot of maintenance.
Can I transfer the bio-media (sponge, ceramics) from my current hang-on filter directly to the canister filter?
Sure, you can transfer media that is already seasoned into the new canister, it will help with cycling. That's the beauty of canister filters, you can put anything you want in them!
Does my tank size matter?
Yes, as with HOB filters every canister filter is rated for tank size and output in gallons or liters per hour. You should get one that would be adequate for your tank size. If you are looking at a 30G tank then you should be covered. I think the smallest canister Eheim makes has about 116G/hr output which would work great for a 30 and suffice for a 55 (even though I would go with a little larger filter despite Eheim's ratings)
How does the water exchange work? I understand hoses are used, but how does the water return to the tank, and how is it set up to create surface disturbance for gas exchanges and aeration
One hose sucks and one hose pumps it back into the aquarium... Its a closed loop so you don't have to worry about a mess if the power goes out or anything. As for the aeration, you can set your input/out hoses to do whatever you want. For example the output could have a spray bar above water and "aerate" in the same way a HOB filter does. In my tank (I use two canisters) I have one canister's intake on a skimmer and the other intake deep in the aquarium. This way I hope to circulate oxygenated / non-oxygenated water as well as cover my bases with tank flow.
I would suggest either of these don't get me wrong there are lots of good canisters out there ( don't want to offend any Magnum lovers out there) http://www.drsfostersmith.com/Produc...3&N=2004+22777 http://www.drsfostersmith.com/Produc...3&N=2004+22777