I've been playing the game for about 6 months now, with 10 or more 80 planet campaigns under my belt, and still playing the regular game.
If you are actually trying to improve at the game, I would stay with the regular game until you can consistently beat it on moderate (7.6) or hard (8.0) difficulty.
If you don't mind playing on lower difficulties, you can just stick with the original game until you feel comfortable with it, then move on to the expansions whenever.