I haven't played Grim Dawn, but looked at some reviews and gameplay and talked to other people who play it. I think it's a bit a question of flavor which one you prepare, and also there's absolutely no way to tell how many people will play Grim Dawn in a few months - given how recently the game came out.
In my personal opinion, Grim Dawn cannot be compared to D3 on PC, but rather D3 on console: it seems very action-packed and quite polished, and short-term as well as casual it will be very enthralling. When it comes to addressing D3's issue though - the problems in endgame and competition, in particular cheating - Grim Dawn will end up being even worse due to the fact that it's already a hacking festival compared to D3 on console.
I think it's a nice alternative for people who - for whatever reason - dislike D3 temporarily or permanently. But it's not gonna be the savior game some people might be looking for.