I have stellaris, es2 and gal civ 3 incl dlc for all.
Stellaris isn't really comparable its real time and not turn based and very much a grand strategy in space more akin to other paradox titles. Especially with the utopia expansion it encourages making your own story allowing you to create your own race from scratch.
Es2 takes a lot of mechanics from es1 and their other title endless legend and aims to improve on them, it's turn based not real time. It's more narrative driven with a lot of background lore to the playable races and race specific quests. The races are all very distinct and have different play styles. For instance the trade driven lumeris don't colonise planets but rather can buy outposts to colonise and can then trade them to other races for money which no other race can do. The assymetry between races is a key selling point to the endless series.
It's shaping up to be a very good game in its own right but there are improvements needed to ai etc. I'd hold off for at least a month post release.