You talk about clutter on the back of the tank, well arent the hoses from the canister causing clutter? If you dont set it up for a sump now, then you never will. Sumps are easier to maintain, run your tanks cleaner, better flow, are cheaper to run, and imo safer for the fish. No telling how much bad bacteria grows and thrives in a moist, dark and warm canister.
Let me paint this picture. It has been 2 months since you maintenanced the Fx6, you just got home from a busy day and now you have to crawl under the tank, disconnect the hoses, drag that huge, heavy filter out, drag it to where ever you are going to clean it. an hour later you have it back in place. So realizing that you are tired and that you have all of that pain in the rear work ahead of you, you decide "Ill just do it tomorrow".
Whereas a wet/dry sump, you come home, you open the cabinet doors, you then remove the top or pull the draw open that houses you filter media, (floss, pads whatever you use), you gather it up, place it into a container so there is no mess, you replace the pads. 10 minutes your done, and ZERO heavy lifting or getting your wifes bathroom or kitchen sink all dirty from the canister cleaning.
Now lets talk flow and power. It states that the Fx6 is 900+gph but this is what the manufacturer says about their flow. "Note: Flow rates were measured with intake and output hoses of the same length and without media." So as soon as you start running that thing the flow goes down, Id be willing to bet that your flow would be more around the 500-700gph or maybe less. Also it draws 50 watts of power which isnt bad for a canister. Now in a sump you can use a RIO 2500 for example it pushes 700-800gph and does not go down in pressure as the wet/dry becomes dirty, and it draws 50watts. Or you can go with what I use and that is the Deep Blue Triton. I use the #5 but the #4 is what we will use for comparison. The 4 uses 40watts of power and pushes 1050gph, all day every day. Heck the #3 uses 30 watts and pushes 850gph.
Also speaking of clutter, with a sump you can add your heater to the sump and not have to have it hanging on the back of the tank with an ugly cord. Not to mention if the heater develops a short, that short is is the sump and not in the tank where your fish live and where you place your hands.
I ran canisters and HOBs on all of my tanks, at one point, my electric bill was over $500/mth. I switched to sumps or wet/drys on all tanks over 150 gal and sponge filters on all tanks smaller than 150. My electric bill for 34 tanks and a dehumidifier running 24/7 is now $240/month.
I hope this helps