I haven't played Gal Civ, but I've played other turn-based 4x games such as Civilization and Alpha Centauri, and my money is on Sins hands down.
I like the 4x-TBS's in principle but they get really boring later in the game when you end up having to micromanage large amounts of stuff. After you've mastered telling your cities what square to farm, how much more fun can it be to have to manage that sort of thing constantly? (You can't trust the AI to do a good job for you with that.) You'll also have large amounts of units that need orders each turn. Also, how much fun is it to tell your settlers to build roads, farm, and build railroads? It gets boring after a while. Maybe that stuff isn't present in Gal Civ, but I can't imagine that the Gal Civ game play, being a TBS, is much different.
In contrast, Sins almost eliminates that sort of micromanagement. I love not having to tell my planets what square to farm, and I love being able to select a large group of units at one time and to queue up orders.
The other big advantage Sins has is online multiplayer against human opponents. A 5v5 on the Sins Diplomacy expansion (Quickstart + Faster speed settings) only takes about 1.5 hours. In contrast, it's nearly impossible to do that with a turn-based game. This is a large issue, because you might eventually get tired of the AI and its lack of the ability to be creative or to use good strategy and tactics and crave real opponents. Online multiplayer also adds team-based strategy to the game and even team voice chat (if you can hook up with friends on Teamspeak or Ventrilo).
With Sins you get to focus on the strategy and tactics and not repetitive micromangement. You're not going to be clicking as quickly as you can to build supply depots and Protoss crystals while trying to click to order the pumping out of marines and dragoons, etc. I think it's an amazing game in that respect. It's a shame that so few online multiplayer gamers know about Sins and that very few people play it in online multiplayer. Sins is an overlooked gem.
