Can you tell me the main differences between those two games? Which one is more suitable for a less casual gaming experience?
Cities XL's big feature was the online multiplayer, but that has been shut down now. Cities XL's gameplay allows you to build cities that are more varied in appearence than those in Simcity 4, with features such as curved roads and different types of zones (linear - think inner city with houses on edge, freeform, curved (good for suburbs), compared to Simcity 4's grid. Some of these can be replicated manually in Simcity, like the linear zones, while others like the curved and freeform zones can't be.
However, Simcity 4 is the more in depth of the two in most aspects with features such as garbage disposal and pre-game terraforming, but there are a few cases where Cities XL has more detail (specific bus routes as opposed to just plopping stations everywhere).
For the less casual gameplay then, I'd go with Simcity 4, but Cities XL isn't entirely lacking in depth either.
Here's a fairly in-depth comparison between SimCity 4 and Cities XL (follow the link to the page for the “long version”):
Note that since that comparison was written, the original Cities XL has been followed by two sequels, Cities XL 2011 and Cities XL 2012. Also, as mentioned by Macha, the original Cities XL's multiplayer features no longer work as the servers for that have been shut down.