I never did play Homeworld 2, is it any good? Homeworld2 is definitely a great game it's the main reason I have hope for sins mp which I havent tried yet. I will say though that once you've played sins, HW2 will really feel dated IMO.
Homeworld is Homeworld. Nothing will EVER come even close to it. All three games are simply godly awesome, and not even sins manage to be as good as any of them.
That said, Homeworld is more focused on tactical combat, where the player have to assume complete, direct control (micromanagement) of each and all ships. While sins is more focused on strategic combat, where the focus is more on the process of building an economy, then a fleet and moving it, rather than actually directing each ship during combat.
For me, it is not a question of "who is better?", rather it is simply a question of "are they all fun?"
The answer is YES, they are all awesome. Both Homeworld and Sins have sucked up a huge numbers of hours away from my life.
My only complaint is that there isn't a Homeworld3.