Pros about Civ 4:
1. I had a great deal of experience with this system.
2. It has Saladin as a civilization leader. (Never played Arabs before, but hey.)
3. Narrative voice acting, previously heard in Sid Meier's Alpha Centauri but never used in a game with Civilization in its title, is provided by Leonard Nimoy. (I didn't know who was Nimoy when I played Alpha Centauri or Civ 4... -_-)
4. Expansion packages are included, so I will get new content if I order the whole set.
Pros about Civ 5:
1. It's sparkling new!
2. It comes out on Sept 1, shortly after I come back from my vacation.
3. It has Wu Zetian as a Chinese leader.
4. The first incarnation of the series where the leaders speak their native language.
I could, in theory, just get both. But that somehow seems very wasteful...