The imperial units are primarily single function while rebel units all have secondary weapons. Imperial units are "cheaper" than rebel units in terms of command points which dictate how many units are available to a player at any given time. The game is well balanced and you can have teams of rebel and imperial vs. rebel and imperial or in any 4 way combination in multiplayer and skirmish modes.
Stay away from Voodoo 2's(they are cheap and pretty available) for this game. You will encounter some graphic corruption and slowed gameplay. Voodoo 3's and TNT's are pretty good.