Yeah, SimCity 4 is highly customizable and expandable, but it is so incomplete to begin with (especially without the Rush Hour expansion). I remember when SC4 first came out, you couldn't even control the layout of the streets. At it's heart, it's a very mediocre game, and only the SC4 community made it a worthwhile experience, if you are willing to take the time to download all the mods and lots and stuff (Road Top Mass Transit is a must).
Unlike SC2K, which is a light-hearted science-fiction game, SC4 was meant to be realistic, but it's just not detailed or complex enough, even when compared to SC2K in some ways (there's only a single water level in SC4, for example, which I find annoying).
So at its core, SC2K is far superior to SC4, but with Rush Hour and mods, SC4 might be the more satisfying game overall, especially after having majored in human geography and taken several courses on urban geography, including urban planning. SC4 just requires a lot more dedication and patience.