SupCom is driven by tactics and strategy on a military level greater than another game ever made. You always need to have a good balance between forces (land, air, sea) and have to manage an economy, which sounds simple, but being able to get an economy that can pump out enough mass and energy to build multiple experimental units in under 20 minutes and producing legions of units at the same time, while making strategic decisions based on your opponents maneuvers has almost become an art form. And this is taken even further in online play, where the game becomes extremely competitive. It's almost to the point that the game is so immersive in online play that it almost doesn't become fun, but an actual battle that leaves no room for mistake.
When Forged Alliance (acronym FA), it revamped the entire game, basically making a whole new one. It fixed almost all the bugs and unbalances, and all changes have been welcomed. So with that said, I really have no negative say on the game.
Sins on the other hand is more based on the 4X play, and almost no tactical play. Players focus more on expanding their empire through trade and economy (real economy, not resource intake like SupCom) and having a fleet to conquer disputed planets and kill other players. The only problem with the ships is that ship (other that fighters) will just sit there and take damage. Also, Sins is so much slower that SupCom, and so less hectic. But the problem with that is that it can slow to a crawl that can make it get incredibly boring, especially ship combat.
I really wish that the ships would fight for position, and that taking hits in certain parts of the ships were weaker --- taking hits in the thrusters would deal more damage, and there would be more armor on the fronts, much like the German tanks of WWII. Having this would make controlling your fleet in battle all the more important. To add to that is how capital ships will simply turn to the ship they are attacking, when supposedly capital ships are bristling with weaponry on all sides, which would make the turning into the ships (disregarding when the capital ships that have cannons in the very front of the ship) rather have no point, particularly when dealing with frigates. With SupCom, it is better to have units always move due to the fact that not all shots will hit them. Shots can miss, leading to smaller amounts of units being able to kill larger ones, where as in Sins, battle is really who has more capital ships.
To add to that, it would be nice to have more defense stations other than the single ones provided, as well as cannons that can fire into the phase lanes to a planet 1 phase jump away so that when you jump to a planet that spams the defense stations, your ships won't be killed by the unmissing fire.
In the end, my choice would have to be SupCom. It delivers some faster and better game play, based more on being able to make decisions rather than just sitting there, and the battles are by far much better. But, were there to be and expansion pack or patch/update for Sins that would make some of the changes mentioned earlier, then the race would be too close for judgement. For now, SupCom & FA, but I hope that Sins will get better.