Buy GalCiv II and be sure you get at LEAST the first expansion. The second upcoming (this month) expansion is gonna be fucking nuts as well so buy it too. That game is much deeper than Sins and will provide you with more time sink. GalCiv II becomes exponentially more exciting with each expansion. The difference between vanilla and Dark Avatar blew my mind, and Twilight of Arnor is adding even more stuff than they did in Dark Avatar.Sins is an excellent game, but it sounds like you are looking for something more like GalCiv. Sins is also in its initial growing pains, come back in 3 or 4 months, and it'll be just as top-rate as GalCiv quality/balance-wise. Be advised that there is no multiplayer in Galciv II.You can't really lose, though, going one way or the other. It's one of the best decisions you'll ever have to make.
I agree with this 99.9%, and only because of one excitable word that I also basically agree with

Seriously, I own both. I play each when I'm in certain moods, and it's very true where it's being said here that one has strengths where the other possesses weaknesses. But also, I can along with others attest to the scale of the improvements in patches/expansions.
GCII doesn't have MP, but the AI is sooo good you won't be able to tell. The game still does things to me I would only expect from another human, and can walk all over me without needing any sort of cheating. Can't put a $$ figure to that in my eyes. However.. other side of the coin, in terms of making soup Sins has only just come to boil and I still see chefs working on it. I actually believe both games are going to continue on and improve far beyond their vanilla versions.
My advice, get GCII:Dark Avatar (because of the comments you made about liking the building part, I'm not so different) and then perhaps save up for SoSE, it sounds as if GCII would suit you just a little more than Sins but both are stellar games and I don't know which I'd part with if I had to make a choice...