Well now,
Been playing GalCiv2 for the last few days, and I must say the game is pretty impressive. Unfortunately, it fall shorts in several areas. Most of my opinions are based on my positive experience with MOO3 (despite the bad reviews and frequent upset fanboy outrage).
Race Configuration
Both MOO3 and GalCiv2 are on par in terms of race configuration. Each has their own unique strengths, but both go equally in-depth and the options are detailed enough. I especially like the advance configurations in difficulty level for GalCiv 2.
Galaxy Gameplay
GalCiv 2 uses a "free-for-all-travel" system, meaning ships can travel from one point to another freely across space. Comparatively, MOO3 uses a "warp-lane-travel" system that limits ships as they need to travel through certain warp lanes (of course you can travel off-lane but that takes forever). In terms of gameplay, it means that unlike MOO3, you can't have chokepoints in GalCiv2, which drastically alters one's strategies. Personally, I prefer the MOO3 format... but to each his own.
MOO3 also featured the "mobilisation center" system which is a plus point for me, as within a short notice, I can deploy a fleet at any planet, thus simplifying logistics by a huge amount. While the system is not as a realistic as the conventional "travel-there-by-engines" approach that GalCiv 2 takes, I still prefer the former for it's simplicity (and convenience).
Technology Tree
I have no idea why every review I read says GalCiv 2 tech tree is "large". By my standards it is "medium"-sized at best, considering that I could only see about 200 technologies (unless there is alot more that I have yet to unlock). Compared to MOO3 which had 6 fields, 50 levels and average of 2-3 tech per level, totalling to 600-900 technologies in the game. However, the saving point of GalCiv 2 is that you can research INDIVIDUAL technologies, which is a big plus. I got seriously turned off in MOO3 when I had to research each level at one go... taking the strategic option out from researching technology.
Empire Size & Planet Count
I really have to admit MOO3 wins GalCiv 2 when it comes to empire size and planet count. In MOO3, on the largest galaxy, I can be controlling 300+ planets in the mid-game while dominating 1/3 of the galaxy (total planet count would be almost 1000 planets). Contrast this in GalCiv 2 where I have about 50+ planets controlling 1/4 of the galaxy (projected total planet count of 200+). Of course, I have yet to colonize the Class 0 planets (which I hope will be possible in the late game).
Ship Design & Classes
GalCiv 2 wins MOO3 hands down. The ability to "lego-build" your ships in GalCiv 2 is extremely satisfying. However, I am very disapppointed that we are stuck with mere fighters for the early game, and do not get proper battleships until the very late game (which I have yet to reach). Currently, I'm still using Frigate-class ships and that is extremely annoying. Contrast this to MOO3 where you start with the Light Cruiser chassis.
Fleet Battles
To make a comparative analogy, fleet battles in GalCiv 2 is similar to Star Trek, while MOO3 is similar to Star Wars. The fleet battles and ship count in GalCiv 2 is slow, and often only a few ships (less than 5) can participate in the battle. Not to mention they can only mount a few weapons (less than 10). Compare this to MOO3 where you can deploy 64 ships in an armada (or 255 ships per armada with the correct mods), and have a shit-load of weapons on each ship. The action is intense (and sometimes mindless too). Also, while there is no unit cap in GalCiv 2, I still find that MOO3 you can field much more ships than GalCiv 2 which is a big let down for this.
Diplomacy
Both game sucks equally in diplomacy. In MOO3 there was very little options you could do in the Galactic Senate, which mainly comprised on the random issues raised and voting for the president. GalCiv2, which touted an advanced diplomacy system, is just as bad as MOO3. So far, I've yet to see any election for the president of the United Planets (not to mention that the president of Galactic Senate in MOO3 has NO POWERS unlike Sid Meier Alpha Centauri). I've also yet to see if we can actually create or select what issues to raise during each United Planets meeting, instead of being randomly generated.
One thing that is actually good in GalCiv 2 is that civilizations can actually surrender unlike MOO3 where you pound them to death. Although, I admit the "surrender-to-another-empire" trick is really pissing me off some time.
Game Performance
For a such a beautiful game, GalCiv 2 surely runs very smoothly on my computer, even with the largest galaxy and by mid-game. Compared to MOO3, which can average to 5min per turn (including auto-resolve battles), the game programming of GalCiv 2 is clearly superior and more optimised, which translates to a better gameplay experience.
Overall Gameplay
The distinction of GalCiv 2 and MOO3 is pretty clear actually. Much like the developers promised, I actually spend quite a lot of time building up my civilization, and less so for my military (until the Torians declared war on me). Comparatively, in MOO3 I am building up my fleet (as I let my governer handle my planet infrastructure - which they are extremely good at, once you start to understand their macro-management mechanism), and literally fighting battles every turn towards the late game (which can be very dry and repetitive).