Quote Originally Posted by Cennyan
Enslaving actully builds you're overall population faster than the occupying. When you occupy you keep the same population in that city, but of course you have a lot of trouble with order. When you enslave, the population is distributed between your other cities. I just went on a rampage through greece and took over four cities in four turns. At the end of the four turns, by enlaving every populace, I actually got four cities upgraded to the next level.

Enslaving is almost always the best option.
That's a very good point. About the only argument against it that I can think of off the top of my head is, the cities that gain population are already behind you. I like fully populated new conquests so I can upgrade and rebuild troops "at the front."