I waited on CIv 5 for a long time. Vanilla was just that, Vanilla. Gods and Kings was a vast improvement, and now I'm playing Brave New World, and I really like it. I would change a couple of small things, but overall, I like Civ 5 better than Civ 4... I know, I know, sacrilege. 

For many people, the sorry state Civ5 shipped as is unforgivable. However, Civ4 release version was just nearly as bad and at the time people claim that Civ3 was superior to Civ4. Civ4 wasn't that great until Beyond the Sword. I personally think 5 is far better then 4 was. In Civ4, There wasn't much strategy to combat other than pile 6 catapults and a bunch of units and ram the enemy army. Then mop up. 

Why not both? Steam offers regular sales, and one is upcoming ("hide yer wallets" and usual stuff), if money is leading constraint you have.

I own both Endless Space and Civilization series, I enjoy both. Not going to argue which one I like more, or walk onto dangerous and slippery path of discussing which Civ is the best, most interesting, or most challenging, etc. Even if they are similar somewhere, they still different.

I understand, games like that (and pretty much everything made by Paradox) are games of "happy retirement", because of all time you could possibly invest into them, but I think having both is the winning move, especially since both hit 75% discount bar in Steam. Okay, probably winning move is to find someone with both and play on his account or use Steam Family Sharing feature for same result, and then made decision, but you got the idea.

I played Civ5 with G&K. It felt bland and boring to me. I just didn't get an "epic" feeling from it. Building wonders used to feel important with civ4. In Civ5, it's just an accumulation of various points, and it feels hollow.

I've never played endless space, but IMO, Civ5 is skippable.