All ARPGs are grindy.
Diablo 3 is definitely more suited toward the casual player. Its easier to gear your character and your end game is more of trying to get as high on the season's leader board. in D3 you can get carried to max level and really start the "end game"
POE is alot more involved as there are more classes you can play, infinite skill trees to use, and then there is gearing. Trying to get the best gear is an endless quest as you can always try to craft/recraft certain items to get the max DPS. POE does not allow your character to be carried to max level, you have to replay the entire story for each character you make.
POE2 is coming soon so you might want to wait for that instead of picking up POE now. POE is also free to play.