Civ IV is more fun but in a different way - it's also far less realistic. You can just run around with 50 tanks stacked on top of each other and rampage through enemy countries.
In Civ V, armies actually look like armies, and positional strategy actually matters.
Well, you have 50 tanks "stacked" in an area equal to the size of an entire city. I don't see how if you are attacking a city that needing to have your archers coming from a different direction on the map from say your mace men is more "realistic".
Civ V is fun, but it has LESS strategy than IV (which some may prefer). Just go to the CivFanatics forums and read what those who own both have to say.