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Whitch is better for a 72 gallon saltwater tank I would love a sump but it won't fit under my stand and I'm trying to keep the weight down since I'm on the 2nd floor
Canisters are superior to HOBs in every way, though the type of filtration needed for a saltwater tank is largely dependent on the type of SW tank you want to have. In tanks with live rock, I believe power heads are all you really need because nitrification occurs in the rock.
Being superior does not mean they are a better choice for all application. There's nothing a HOB can do better than a canister. But, like I said, SW filtration is largely dependent on the type of SW aquarium and without knowing that it's hard to give a concise answer.
Any pics of the stand? A small sump is better than no sump.
I wouldn't do either. If you really can't fit even a ten gallon as a sump do lots of live rock, a powerhead, and hob huge and skimmer. These would be better than a regular hob filter or canister.
Weight shouldn't be an issue. I have never heard of a tank going through a floor. People are fat, have waterbeds and gun safes, the floors are made for it.
Skimmers don't work in freshwater. Freshwater doesn't allow for the size bubbles needed for a skimmer to work efficiently. The only exception I have ever seen is in a store that did a lot of koi which are routinely kept with a lot of salt (at least at wholesalers and retailers). The salt is enough to allow for bubbles small enough to effectively skim (and the cost of the koi in one stock tank being in the tens of thousands makes the investment in the 6' tall skimmer worth it).