I’ve played a lot of both so I should be able to answer your questions.

It’s much simpler mechanically but the the build diversity is large and there’s plenty to dig into. In my opinion it strikes the perfect balance. The end game in quality is probably slightly better than D3 but no where near as good as PoEs end game. PoE has simply built upon itself for so long it will always top the market there and no other ARPG will catch up until GGG stops iterating.

In general the progression feels better to me because I’m able to understand it every step in the way. I make my own builds that clear ultimate and that empowerment makes the progression worth a lot more than following a build guide for PoE.

Beating ultimate and clearing every boss is basically finishing the game and it can be done with very little grind or a lot of fear grinding depending on your build. Totes up to what you want out of the game. There’s also tools to let you get alts caught up faster so you can be into the hard stuff. There’s some infinite grind methods available though if you want them.

I don’t really know what you mean by casual friendly. There’s no leagues so there’s no fomo and you just play when you want. There’s no leash pulling you forward like PoE.