i have tried keeping marine fishes n corals using canister...a few things u have to note.
there will be micro bubbles coming out from yr cannister (normally, this dun happen for freshwater), somehow u will experience this in marine.
u need to clean yr cannister at least once every 2 weeks...a lot of detritus will accumulate at the bottom.
and no matter how hard u clean yr cannister, once u switch it on again, dirt will come out into yr main tank.
without a sump, your equipments will be exposed...aesthetically, will not be nice and pleasant to the eyes.
if u want to sustain this hobby for 3-4 years, it will be better to have a proper set-up with sump tank and overflow system....if u divide the overall costs over 4-5 years, it doesn't really cost u a lot and the tangible and intangible benefits u can get are enormous vs a non-sump tank....
However, if u have already committed to the set-up, its ok...u can still sustain with less sensitive fishes and corals...and probably, when u r deeply poison...can consider upgrading....haha...
cheers mate!