I personally did not play much of Sins of a Solar Empire, but it's a very beautiful game with a very slick UI and controls.
Although I bought both Homeworld games new, I've never played much (mostly skirmishes). A bit hardcore for me, but they are considered classics for many reasons.
SC2 has a pretty neat single player campaign. It's a great story, especially if you familiarize yourself with the backstory in SC1. It's also got a ton of unlockables, two completely different customization trees (per expansion) , which each have tons of awesome units, and lots of lore, clickables, dialogue and other fun stuff in between missions to engross you. I wont lie, I found the actual campaign gameplay very simple, and I've never really been a fan of playing the multiplayer, although I love watching it played.
Everything else I can think of, that I really liked, didn't have much story unless you made it yourself. To be honest, I think Sins almost wins by default for new strategy players. It has the best interface and visuals I've seen, it just came at a time when there were a bunch of super complicated grognard or roguelike 4x games on the market and I didn't have time for it.