Smac = Gameplay, intricacies, strategy, diplomacy, more governments, more units (customizable), more faction personality, immersability, OTOH complex so learning curve is high. (back when released there was so much forum activity around SMAC that learning the game was fairly easy. Now adays newcomers have to kinna fend for themselves so the learning curve now is much greater than when initially resleased)
Civ3 = Eye candy, great starter game for those who are new to the CIV genre, some attempts at new game features culture and resources (each of which are of questionable implementation).
Each have some merits but without question my fav is SMAC.
P.S. For any of those who have interest in taking up SMAC, feel free to post question etc. in the forum they will be answered.
PPS. Selous below. I agree long timers used to CIV concpets will find the learning curve relatively short. OTOH newcomers with no CIV backaground will prolly be extremely daunted as IMHo SMAC is the most complex of the CIV-like offereings to date.
PPPS. Interestingthat the comments and polls show SMAC doing so well. I fully expected in a CIV forum that SMAC would fared miserably. Although CIV3 leads in this polling I think the results show very favorably for SMAC (considering the forum the poll was placed in).