You can use this page to see how many miles you get on each partner in each eligible booking class
http://partnersearch.ehn.klm.com/af/us_en/

Flying SU you get 175% in C and J or 150% in D or I class.
Flying KE you get 125% in J, C, D or 100% in Z.

So you're looking at something like 13500 or 16900 mi return if you go the KE way and 13400 or 15600 mi if you choose SU. All in all it's a wash

Only you can decide how much flying a better product (KE) justifies spending longer in the air. Or you could fly one on the outbound and the other on the inbound and share your thoughts on Flyertalk as to how the two compare.

P.S. Personally I find J on Aeroflot's A320s more than adequate on shorter flights and have no experience with KE on longhaul. But I flew one of their older 747s in J on mid-haul back in 2008 and it was absolutely nothing to write home about.