GalCiv is a TBS, kinda like Master of Orion, whereas Sins is technically an RTS but tends to not be too bad of a clickfest.  Sins is unlike any other game I know, but does have some few small similarities to Homeworld in its 3D RTS-in-space nature.

Why not try Distant Worlds at Matrix Game.  It is RTS with pause.  Distant Worlds > GalCiv2.  Sins is fun if you don't have time to spend playing game.

The expansions for Gal Civ 2 are rather nice... I love the game, but really have to say I still liked MOO2 better because of tactical battles... It was so fun using strategy and tactical finesse to beat the computer in MOO2, I could have 20 ships and take out hundreds if I did it right. In Gal Civ, well it just wasn't quite as fun IMHO. Still a good game though.

Sins is something I'd like to pick up sometime, I played the demo and enjoyed it, the whole space genre is one of my favorite. Just picked up AI Wars, you ought to try that out as it is rather fun. They have a demo too, takes a while to get the strategy down but they have lots of documentation and are constantly updating the game.

Sind of a Solar Empire is more like Haegemonia if you know that (But with much much bigger battles). Great game. Especially in Multiplayer.

Gal Civ is more a Space Civilization Master of Orion mix but sadly with the lag of a multiplayer mode. So total different games. If you are more the Multiplayer competitive player then buy sins of a solar empire. If you are more a solo gamer who likes Turn based games then buy Gal Civ. But if you love great sci-fi games in general ^^ Get both