Actually, I haven't noticed any fundamental differences between the original Homeworld and its sequel. Homeworld 2 is a bit more fast-paced, it has a better interface, the balance has been slightly altered and that's about it.
What?!
Homeworld 2 is a bit more fast paced and has a better interface, and it is better for skirmish mode, but you don't play Homeworld games for skirmish mode because it sucks. If you've played both games, the campaign for the first game is leaps and bounds better (better music, atmosphere, story, etc) than the one in Homeworld 2. This is because HW2 got shipped as a fetus (before it was due) and they had to rush things.
Try the original, Levski, it's one of the classics. The interface is a little bit harder to use, but not that much, and it is balanced (or at least seems to be) much better than HW2.