[CarlJ]

You will probably want more water circulation in the tank than either of these filters will provide by themselves, and more turbulence. Think at least two powerheads the will at least partially intersect. As for the filtration- your rock will provide much of what is needed from a break down of waste standpoint, so use whatever you feel the most confident in keeping clean. I'd run only disposable types of filter floss or sponge, preferably of a type that you can manhandle a bit under the tap (if safe) or in discard water a few times to extend the life.

With a FOWLR (fish only with live rock) tank, you will be able to get by with the current filters while you get comfortable and watch your knowledge base and goals evolve. Ultimately, you will probably remove them both. With a 55g you can probably even worry a bit less about nitrate for awhile and even only run the skimmer part of the time to extend the time between water changes a bit. Likewise, don't worry about the sump until you reach a level that you know exactly why you want it and how you will use it. I would venture that most of us use them to make things more complicated while only really benefiting from a bit of extra water in the system.

Don't focus so much on % of water you change, but rather target siphoning off the detritus, and go with a basic salt mix until you are ready to add the more delicate creatures that will consume more of the minerals in the water as part of their routine, then upgrade to one of the more expensive ones that contains the extras of the things you need.

 

[CarlJ]

It's only a nitrate factory if you don't keep it clean. If it helps you take the crud completely out of the tank, it may be your friend. (ie trapped in the filter is not really out of the water column.) If you have waited for the tank to cycle before adding livestock, there really should be no ammonia to remove and the carbon is only necessary if the water you are using still has something you want to remove, or if you had to treat with something you need to take back out.
If you plan to keep invertebrates, try to avoid adding copper. Expect to get something ugly as part of the saltwater cycle. Although the nitrogen cycle runs about 30 days, the saltwater tank will take about 6 months to really be settled in. You'll likely see at least a couple of: diatoms, cyanobacteria, slime algaes, spirobids, pods, tube worms, and dinoflagellates. Will good use of your siphon hose, most will clear up pretty quickly.
The filter pad is the only thing I would keep long term, and I would clean it 2-4 times a week once it actually starts catching anything. If you are unsure of the carbon, keep it for a few weeks to be confident nothing was added unexpectedly for peace of mind.