canisters are not the best filtration... the best is natural, or biological. a canister filter will work, but does not do best for salt water.. your best bet is a refugium! If you are looking to get into FOWLR, or fish only with live rock, then a sump would do fine.. i had a canister filter and removed it.. spits out micro bubbles into my tank.. dont mean yours does or will, but i heard micro bubbles are bad .. A sump will also allow for better oxygen exchange and a fuge, or refugium, will house many beneficial bacteria and algae.. Algae helps to removes nitrates the natural way. Alot of people go with the sump/fuge together. but from what i have been told, if you are looking to setup a reef tank, u will need only a fuge.. sump/fuge also gives u a place to put your heater and skimmer so there is less hanging on the tank if yours is not drilled.. hope this helps you make your decision.. i wouldnt buy a canister for the tank, but if u already have it, might as well use it, but watch for micro bubbles.
you can put your heater and skimmer in the sump and have live sand and rock in your fuge.. This will help the natural biological filtration system and will be very beneficial to your tank, as well as add water volume.. I personally would go for the 125 gallon with a 30 gallon sump/fuge. they arent hard to setup... there are lots of threads about em if you are interested in looking into them. They are sold already assembled, or check the DIY (do it yourself) for plans on making one..
crushed coral tends to increase nitrates and you will eventually switch over to live sand, so id say go with the live sand.. and do not cycle with fish, everyone always regrets it.. may the brave soldiers rest in peace!
