Xiara, I've got both. I recommend Sins! For a horde (sic) of reasons. First get Impulse from www.impulsedriven.com where the full Trinity Edition is now 39.95. You can download the Sins demo with Impulse too.
Sins has the more active community and is a rollicking good game! And with an active modding community too. Play it as it comes at first, but there are a lot of ways to jazz it up.
GC 2 is very good also, a lot more strategic depth lots more screens of nard to look at ... but it also requires something approaching thinking. And thinking makes my head hurt!
I did not like Sword of the Stars. More tedium interrupting what I really like to do which is get my fleets out there blowing s**t up.
Remember that the Civilization 5 demo will be out on the 21st of September!