Let's put things into perspective here and look at the facts....
Sins of a Solar Empire was released in February of 2008...since then, it has had five and a half years to fully develop, enhance itself, and utilize player feedback...as of now, we are looking at a game with 3 expansions and 1 paid DLC...Endless Space was released in July 2012...the game hasn't even been out a full year and hasn't released a paid DLC or an expansion yet (though its first is apparently due out this summer)...
Rebellion (which came out June of 2012, a month before Endless Space) is a $40 game while Endless Space is a $30 indie game...Stardock and Ironclad, while small companies, have more resources and money than Amplitude Studios...Stardock in particular has/had released several games while Endless Space is the first release for Amplitude Studios...
The original Sins of a Solar Empire had 500,000 sales by September of 2008 (7 months after release), and 200,000 of those were in the first month alone...Rebellion had over 100,000 sales in its first month (can't find a more recent sales figure)...by March 2013 (8 months from release), Endless Space had sold 300,000 units...note that Rebellion and Endless Space are basically Steam sales while the original Sins had a variety of distribution methods (I got the original in a box as a gift)...
Now for Metacritic Scores:
- Sins of a Solar Empire -- 87
- Rebellion -- 82
- Endless Space -- 77
Conclusions:
Sins of a Solar Empire outdid Endless Space both in sales and reviews...Rebellion also outdid Endless Space in reviews and probably sales (don't have good numbers to compare)...what is interesting to note is that the original sins of a solar empire seems to have outdone Rebellion both in initial sales and review...
Despite the numbers, it is important to consider my first 2 paragraphs...Rebellion has had several years of vetting, criticism, feedback, and enhancing...Endless Space hasn't even had a full year....the developers of Rebellion had a name for themselves (especially Stardock) before even original Sins, and certainly had an established player base to sell Rebellion to...Amplitude Studios is releasing its first game and has no benefit of any prior reputation or established player base...then there's the price difference, and if you argue that Rebellion is only $30 for Trinity users, I will defer to the 4th sentence in this paragraph...
Metacritic is pretty reliable if you are looking for a ballpark idea of quality, but a 5 point difference is by no means telling...if you read a variety of forums, both games have their shares of critics and fanboys...of course more people here will prefer GC2 and Sins, and if you head over to Amplitude's forums you'll see people discuss how much they got tired of those 2 games...
I think it's fair to say that when you consider where each team of developers is coming from as well as how long each franchise has had to develop, it's just not a very fair comparision...Rebellion definitely edges out Endless Space, but we're comparing the end of a franchise to the beginning of another...I'll reiterate the large difference in repertoire and resources between the dev teams....
In my personal opinion, I think Endless Space has a very bright future and a few years down the road I have no doubt it will be a very good contender for best current 4x game....only time will tell which game is truly best...
By the way Xathos, yes SotS II had a nightmarish release, but Rebellion also had major stability and balance issues...sure, Rebellion got them all fixed, but so did SotS II...and SotS II wasn't using a 6-7 year old engine...