It seems like this thread is already past this point, but SMAC is one of my personal top 10 favourite games of all time. Maybe top 5.
SMAX is much less well thought out compared to SMAC. The factions in SMAC all make sense (and despite what another poster said about spartans being not-so-well thought out, I personally find it easy to believe that there would be a "soldier's soldier" type leader, and that her bonuses reflect such a person). This is because they represent common ideologies/archetypes:
Lal: Democracy "The bureaucrat"
Santiago: Might is right "The General" (or colonel, whatever)
Yang: Collectivism (The Group over the individual, etc) "Chinese Communist"
Morgan: Financial Power "Fatcat"
University: Knowledge "academic"
Gaian: Coexistence/Tolerance "The Hippie"
Believers: Religion/God "Bible-Thumper"
As compared to:
Pirates: Plundering(?) "The pirate" (must be... almost 1000 pirates left in the world... and apparently ALL of them made it onto the unity)
Data Angels: Hacking. "The hacker" (See above)
Free Drones: Mediocrity/Freedom (?). "The talentless normals"
The Cult: Copying Gaians, then telling them they aren't "Gaian" Enough. This one is easily the stupidest. There already was a faction that was obsessed with planet. Also, according to the background, the leader of the cult was "found on planet" so why does my turn start with my pod crashing into planet? lol....
Cybernetic Consciousness: Knowledge... and putting shiny things in yourself. "the borg." This is supposed to be a sequel of sorts to the "space race" victory in civilisation, and there are no cybernetic type techs in those games. And none of the other factions have anything like that. And there favourite Policy is WAY down the tech line, because cybernetics is very far away tech wise. So how are they already cybernetic????? This faction would make sense in all sorts of sci-fi scenarios... but not this one. Also, they are clearly just University+, which kind of nullifies the point of playing University ever again...
The alien factions: Obviously, they are amazing. Play them if you want to win, no challenge at all.
Worst of all though, you can in fact play any of the new factions against the old, and expect to win. Why? Because in the original, it was a major bonus for the university to start with a second tech... and every new faction starts with at LEAST 2 techs, and the aliens even more.
Things SMAX got right: Artillery works now (it was completely broken in SMAC). I personally like the extra couple of techs it adds, as there are now multiple ways to get to str 6 attackers (which is more or less where human v human games stop. MAYBE fusion laser for str 8, but all in all air power and/or fusion power tends to finish human games, based on the handful I've played). A couple of secret projects (it's super cheap, but I love the cloud-base academy. But it is definitely super cheap).
The other stuff (fungal towers, spore launchers, etc) I could either take it or leave it, no cares either way.
One of the main problems with this game is challenge, though. I have made more than a dozen extra factions, (Ferengi, Borg, The Empire, The Rebel Alliance, etc), and most of them are over-powered (the ferengi get +2 economy!), and yet they still will get crushed by me on the highest difficulty, while I'm playing as one of the original 7 factions (which, as already stated, are weaker than any of their SMAX counterparts).
It seems like a negative overall list of pros and cons.... and yet I'd still recommend them both, since it probably is only 10-20 bucks for the set.