If you're not too familiar with 4X genre, then this game is a good one to get started. I started with it and was thoroughly hooked.
But if you only have time for one of them, then ES2 is a better choice - it's more polished, and is an overall improvement of almost every aspect of this one. I find it to be more replayable than ES1.

Both games are very different some would argue ES2 is more advanced, has more features, more DLCs...more content. Others might say ES "captures the games essence" very nicely in a way that ES2 does not...I am one of those people.

Not trying to sell you anything either. Here is a suggestion.

Amplitude, the company behind Endless Space is featuring a website (https://www.amplitude-studios.com/) that allows you to create an account there. If you do you used to be granted Endless Space with all its expansions for free as a welcome goodie.

That seems to be the safest way to figure out if this is for you or not, zero personal financial risk. If it scratches an itch then ES2 might be even better. If its not you ll just deinstall and forget about it

No way to claim free copy on the site, i've claimed the endless space 2 addons, but there is no button to claim the endless space definitive edition