Which Sunsun canister filters for a 200L tank?

Hi everyone. This is my first post....
I am an Australian but I currently live in Shanghai.
I have 4 x 4 1/2 inch red melon discus at the moment.
My generous friend is giving me her 200L tank soon but I need to sort out filtration.
I am buying 2 more discus the same size along with a few tetras. A nice 4 inch discus here costs just $7 AU by the way!
The tank will have a thin layer of gravel and driftwood.
I'm thinking either 2 x Sunsun 302's (1000 l/h each) or one Sunsun 303B (1400 l/h).
I can buy the 2 x 302's for the same price as the single 303B. The 303B has a lot more filtering surface but I'm guessing not as much as 2 x 302's.
Any recommendations on the l/h flow rate for discus?
Also where would you position the inlet/outlet for the 2 filter setup?
I've read too that canisters may put an oxygen load on the tank so do you recommend an aerator?
I'm trying not to use foam bubble style filters as they look horrible imo.
Any thoughts greatly appreciated.

Sorry about the double posting guys and gals, my mistake

hi mate,
2 302's would likely be better than 1x 303
id put an in let in each of the back corners and the spraybars just under the water surface so you get good surface movement from back to front or side to side wont make much difference.
aerators work by breaking the surface tension of the water and allowing gas exchange, I wouldn't bother if you had the spray bars configured as I described, if the water surface ripples you'll get plenty of air in there.

Thanks a million ILLUSN!