If I remember correctly,
PRO: Better boards, Pro Taper handlebars, walker evans shocks, belt drive, 155 track, 20 lbs lighter
RMK: Old school boards and handlebars, RydeFX shocks, chain drive, 144/155 track
Not sure if the driveshaft is different.
What he said for RMK vs. 13-15 Pros, plus:
RMK driveshaft is steel. No collars needed. Same brake caliber as pro, solid rotor (the 11/12 pro drilled/vented/lightweight rotors bolt right on a RMK). Different master cylinder on the bars, of course, with old school controls from 99. Different speedo pickup.
RMK has longer front shocks - not interchangeable with 13-15 pros.
RMK has aluminum overstructure, rear bumper vs. CF on pro.
RMK has welded lower a-arms, vs. glued on pro. SAME BOARDS - they are not different, same tunnel, same track, same drivers, same skid geometry and design.
RMK has no fancy stitching on seat, single material type.
RMK has front cooler, always active on a bypass.
Weight difference for 155's is 17lbs dry, maybe about 10 more wet on the snow - front cooler eliminates a big chunk of ice at front of tunnel - I've owned both. The ice chunk is real.
RMK has four additional idlers on skid, and does not come stock with scratchers. You will still want scratchers.
RMK has a solid snowflap, no holes.
Same motor, intake, exhaust, and clutches. Get a 155, so much better all around. I would think the shocks will be shot within 2 seasons, less if you're a hammer (maybe a ride). Sprung softer than a pro, also. These shocks are the Achilles heel of the sled if ridden much, or hard. Lots of options, tho, from used takeoffs to wild custom valved. MH will fill in the rest that I've forgotten.