I just played the Star Wars: Empire At War demo. Aside from the space battles (which are pretty cool) I remained largely unimpressed. The land battles seem a little 'frantic', as if we have little or no control over the action.
I'll try the X3 demo next.
EDIT: Just played the Desperate Space demo. It's good, but I wish they let you get to the cooler weapons before the demo timed out :(