In one sentence :
Cities XL is to Simcity 4 what Dofus is to World of Warcraft.
On one side, you've got an American game with a sizeable starting budget, and polished game in the end, that welcomed one or more addon, though
On the other side, you've got a little French company that struggles for their first games, who are somewhat lacking in depth.
(disclaimer : I've passed way too much time on all four games )