Civ4, but both are superb, sims city 4 is the best SC game ever imo as it's finally a true simulation with good traffic management.
Civ4 is very good, better than 3, I still can't decide yet if it's better or worse than civ 1 and civ 2.
Civ4 will take more of your time and will last longer unless you really love planning and designing.
Go take a look @ the main fansites and see what looks better for you:
SC4:
www.simtropolis.com ( massive site, active forums, new mods every day)
Civ4:
http://www.civfanatics.com/ (big forums, quite a lot of mods, a lot of nice game journals to read and a lot of guides, great site&forum imo)
http://apolyton.net/forums/forumdisplay.php?s=&forumid=176 ( dont know this too well but supposed to be the 2nd big civ4 site... )
Personally if I'd have to pick: Civ4, but I wouldn't manage without both tbh, essential games for a pc gamer imo.