Canister are more expensive up front, but tend to be longer-lived than HOB filters, at least if you buy a quality canister (e.g., Eheim). Many folks have Eheims running daily for several decades.
Canisters are somewhat more flexible in how they're packed and used. You can have canisters packed per manufacturer's specs, in which case they provide mechanical and biological filtration (and chemical filtration, if you choose). In these cases, maintenance may be an issue because the solids that get trapped in the canister really should be removed (by cleaning the canister) or they can break down and contribute to nitrates and other dissolved compounds. A prefilter can help prevent solids from entering the canister, thus extending the time between maintenance.
Canister packed as bio-only filters (i.e., packed only with bioballs or ceramic noodles, prefiltered with a sponge) provide great biofiltration, a little mech filtration, and require very little maintenance.
In general, canisters are quieter (my Eheims have to be touched for me to be sure they're running) and less visually obtrusive. You also have a bit more flexibility on how intakes & outlets are configured.
I'm sure there are other points, but that's what I can think of at the moment...
HTH,
Jim