Shoddy rip-off of Master of Orion II; worse than the ten year old original in almost every way. Buy MOO2 on budget.
GalCiv2 changes the following (and *only* the following):
A simplified (read: more restrictive) economics system.
Almost all of the fun combat technologies are gone. Combat is just a simplistic weapon vs armour calculation.
Fighters, planetary defences, orbital space stations, planetary bombardment, all gone.
GalCiv2 manages to mangle MOO2's UI even while attempting to clone it; it takes too many clicks to get things done, and doesn't display information as helpfully as MOO2.
GalCiv2 adds a mediocre 3D engine that you'll never see, as full 3D combat scenes take far too long to resolve and are thoroughly uninvolving to watch, unlike MOO2's bold and frenzied 2D combats.
I'm not saying that GalCiv2 is worse than MOO2 *for it's time*, I'm saying that it's a worse game in absolute terms. It took its developers ten years to rip off MOO2, and they didn't even do a very good job of it. If you like GalCiv2, you'll love MOO2, and you'll only need to pay a fraction of the price to get it.