Maplestory limits you do specific dailies, events, etc. You could choose to do them or not, but at the end of the day you will be progressing much slower than someone who did them. You have so many dailies (still more to be added) to a point it takes HOURS to finish your "dailies". By this rate I'd say dailies cant' be completed for a person who has more than just Maplestory in their lives. I'm mainly referring to Reboot here..
As far as D3 goes. You can always go on and off anytime you want. As long you're not competing for seasonals, then it is much better imo. You can progress however you like, but in the end you will still reach end game and patch updates always revive certain set items so anyone can be OP in little time tbh.
D3 is less grindy compared to Maplestory because at least you're not limited to doing bosses just 2 times a day.
If you even like d3 a little bit, definitely try out path of exile. It is so much better than maplestory. If you were to play path of exiles, I would recommend you wait till march for a league(season). These leagues each last for 3 month then all your league characters move to a trash server. It would be a lot easier to start in a new world.
I've been playing some MS Private servers, but primarily I play Path of Exile for ARPG strategy. Went back to maplestory for a while waiting on 3.0 beta, Diablo 3 opens every once in a while on my computer but I feel like the game gets boring after the first ~48 hours of playtime on a new season start.
As some others have said before though, it's definitely comparing two completely different games.