Both CivIV and SMAC/SMAX are great games.
But all in all, if I had to vote, I vote SMAC the best of all
And SMAC had a lot of influence in CIV. Some of these influences came in Civ III, but mostly in a bad performance:
Terraformer - Worker: the concept of different units for settling and working is taken from SMAc, but in CIV III, you had only the "terraforming" choice of iirigating or mining

, much better in CIV with cottages, windmills, lumbermills etc.
Crawlers - Colonies: The colony was another bad performed featere taken over from SMAC (working tiles outside the base radius). Sadly not adopted to Civ IV. I would have liked a Colony unit similar to the Smac Crawler which could work tiles outside of the city radius.
SMAC special abilities - promotions
Social Engeneering - Civs
Features of Civ IV and SMAC in comparison:
Graphics: Civ IV winner
Units: SMAC winner. Smac design workshop with special abilities and morale system versus promotions only
Strategy: SMAC winner. SMAC wins battles with strategy, CIV with number of troops.
Spionage: SMAC winner
Sea warfare: SMAC winner
Artillery concept: SMAC winner
Air warfare: SMAC winner (but this may be my personal opinion)
The factor of personal infliction is much bigger in SMAC. All You who post in the "I hate Montezuma/Isabella/Togukawa"-threads - try a Smac Game and learn about Yang, Miriam or Morgan

than You really learn to HATE
All in all, Civ IV is much better then Civ III, which was the worst of the whole series, and also better than CIV II, but SMAC/SMAX, although "old", is still the TOP for me.