I originally heard a lot of buzz about Sins and went and bought it; fell in love with it. It's easy to get into even without reading the manual. The tooltips are great and it's not hard to figure stuff out after a little trial and error and a few games. I had been an exclusively World of Warcraft player for three years and hardly played anything else, but when I finally left WoW and picked up Sins it filled the void. I was impressed with the design, lack of micro-management, zoom capabilities, immense battles, etc.
I started getting the bug to find other games by this Stardock company and something else space based and found GC2 (along with all of the great reviews). I bought GC2 along with the two expansions and have really enjoyed it as well although I will say that it has a bit steeper learning curve then Sins as do a lot of TBS games. Although in my opinion, GC2 is easier to learn than Civ4. It is a 'deeper' game than Sins in that you control many more things in your empire and there are many ways to win besides just warfare or making alliances.
A quick compare:
Visuals: Sins feels like Windows Vista with shiny graphics and buttons. The space and stars look amazing as well as the ships. GC2 feels like Windows XP with good visuals and a pleasing interface but nothing really 'stunning'.
Campaign: Sins doesn't have a campaign. GC2 does have a campaign.
Multiplayer: Sins has multiplayer (I've never played multi as I don't have the time). GC2 has no multiplayer.
User friendly: Sins is easy to get into without reading the manual. The tooltips are descriptive without being overly so. GC2's tooltips are not the greatest. I wish they would be but that's my preference. I enjoyed Civ4's tooltips that included helpful hints within them. GC2 doesn't have that. I was forced to read the manual which is fine but when I start a new game I usually try to rely on tutorials or the campaign without having to read the manual; again, just my preference.
Story: There is a story in Sins somewhere. We're given the 'setup' of how the three races came to where they are now but not much else. GC2 has a great story and along with the campaign you get a good feel for the races.
I currently alternate between the two games and while I ultimately find GC2 to be a 'deeper' game that I can lose myself in creating a story or playing the campaign, I enjoy Sins for its great visuals and immense space battles.
If I had to pick one I would probably go with GC2 mainly because I play pretty much single-player nowadays. But I would save my nickels, dimes, and pennies like mad to eventually buy Sins. But that's my preference.
