I have to admit, some of what they've done with this game has really added to the experience. Every time you scout a new system you stand a good chance of discovering something, which helps to encourage exploration. The nebula in the background look beautiful (can we add more??! I love more background flavor!), and the new music add to the experience.
Funny thing is, it still isn't as gripping as Stardrive for me. It's fun, but the combat and diplomacy of Stardrive are far superior. If Endless Space had better diplomacy, it'd be a good turn-based alternative to Stardrive. As it is now, however, it still feels like it's playing catch up to Stardrive, and SD isn't even complete yet.
TL;DR: Endless Space is improving, but Stardrive is already more fun for me. Will it ever catch up? With the emphasis being on combat for their first expansion, it might still get better, but without a better diplomacy system, it still falls short. I can never go back after seeing the beautiful race diplomacy screens in Stardrive.
ES has a similar development history as SD, except that is supported by an experienced team and not by a one guy coding army; as zero splendidly does. They both had lengthy beta periods, heavily involve the community on design choices and are set to surpass previous 4x games easily. ES does it better IMO since the Gamestogether system allows for the fans to quickly and easily suggest designs(either art, text or gameplay), granting them a priority for the devs by popular vote.
Regarding the actual game, I have to say that ES is totally different from SD; it's a whole other itch to scratch IMO. Not only due to it being turn based, but also due to design choices that flesh out the visual experience; over frantic gameplay in real time. The constant patching, with new features, plus the four free addons(we are talking significant content, not like big patches); keep making the game better with time.
Now I have to disagree with you on the Diplomacy aspect, it's not that it's better than SD, it's just that SD is far from complete; so many diplomacy features are not working correctly. Nevertheless the alliance system in ES, with member voting among other things, it's better than SD system IMO. Although the ES system is simple, it does work great in MP; which is something SD has yet to test.
Finally the main problem with ES is the combat, that in theory(SW Endor like battles with strategy decisions before and during the battle) sound great; but in practice it's not quite there. That's why they are working on the first expansion that will focus on making combat much better, with the addition of strike craft, greater strategic actions mid battle and an even better cinematic focus.
Finally I play both games a lot, although lately SD has been getting most of my attention; since it's finally stable enough to really flesh it out. The combat I have to admit is what keeps me coming back the most, but when I want to play a more role play oriented game or peaceful species, I generally go back to ES. Nevertheless, as I have other games(like PS2, COH or TW2) to play plus sadly something called life as well
, neither of the games get the amount of attention I'd like to give them.