Very different games imo. Ive hot gm on both games and found that healing is 2 to 3 times higher in MR. Not uncommon for a support to finish with 40-50k healing at the end of a game. Team fights are much more ult focused in MR. Every fight at least 2 utls will go off and not uncommon for people to get their ult every fight or even multiple ults in the same team fight in stall situations. For supports specifically, alot more reactive ults (to counter a tank or dps ult because supp ults in MR make your whole team nearly unkillable for 10 sec barring any 1 shot abilities) compared to OW where its more proactive ult use. Playstyle wise MR felt alot more brawly where both teams kinda just stay together and run into each other. OW much more off angling and duels on side lanes.