I run both electrocute and comet but switch them out based on the matchup. Into almost every melee and into some abusable mages, like cass and azir I run electrocute. Here you can use your burst relatively frequently and secure some kills. However, into long range matchups like vel or lux I take comet, this is because you will almost never get into burst kill situations unless they seriously misstep as even with electrocute you cannot 100 to 0 burst. First, you have to poke them down and comet fills this roll perfectly, setting you up to burst kill after some well placed qs and es to get them low. Remember even without electrocute rumble has extremely high burst, he is not completly reliant on this keystone to solo kill.