My $0.02 are that sumps are generally very easy to maintain. But you can make them as difficult and complicated as you want. I think the advantages way outweigh the disadvantages. 2 FX6s add conservatively, 8-10? gallons of additional water to your 'system'? A 55 or 75 gallon sump on your 240 would add three to five times that volume. More water = more water stability. You could keep your FX6s, and add them into the sump and therefore remove their intake pipes and supply tubes from your display which makes for a cleaner appearance. They then don't have to work as hard removing debris, and can be loaded carbon/zeolite/phos-ban to help polish your water, for example.
The weekly maintenance is really only cleaning filter socks or pads. Monthly maintenance could be as simple as detritus removal from compartments and bio-media rinsing (if it accumulates debris). The flow question depends on the size of the pump you use.
Any improperly cleaned fish system will inevitably produce an odor. Keeping to your daily/weekly/monthly cleaning schedule will keep any decaying organic material from accumulating and prevent any smell, as I'm sure you're well aware. I've gone weeks w/o changing socks on my 180 and haven't encountered any odors...the wife would also agree.
There are plenty of ways to mitigate the sounds. Pump noises can be muffled. The falling water sound of the overflow(s) produces the loudest noise, but even they aren't that bad and can muffled as well. My tank isn't in a high traffic area and the ambient, white-noise it produces isn't offensive.
Are you opposed to drilling your tank? If you're getting the results from your FX6s that you want, then I suppose there is no need for a sump. It could be a fun project if you have nothing else going on.
