Yeah you can't really compare GalCiv to Sins. They're both great games, but they play differently. I do hope you enjoy turn based strategy as well as RTS. Because $99 is a lot of money, especially if it's spent on a game you're not going to enjoy.
Of course there is no guarantee that people who like the GalCiv Series and TBS games will enjoy GalCivIII but I'm betting there is a very high probability that I will.