freshwater and saltwater are two completely different things.
generally speaking, freshwater filters are to house benifical bacteria and keep the water from being stagnent. I personally only clean a hang on back filter when it overflows (you'd know what im talking about when it happens)
as for salt water, i recommend NO filter, why? because it traps poop which becomes a nitrate factory. in saltwater systems nitrates are bad (as well as fresh in dangerous amounts) but with live rock and live sand, possibly a sump w/ protien skimmer and cheato algae in it, along with water changes weekly obtaining 0 nitrates should not be a difficult task.
im unsure of the size of tank your planning on using, but generally its easier to go larger (except more expensive) as water conditions will be more stable. having good circulation and enough established live rock should be good enough filtration.
you CANNOT house as many fish in saltwater tanks because of many reasons. territories, oxygen content isnt as great as freshwater, space (alot is taken from the live rock) compatibility and so forth. PLEASE do more research
before spending your hard earned money. it will save the lives of the fish as well as the hole in your wallet, not to mention the headache that comes with it.