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”):
The short version first:
Sim City 4 has been around for ages, and the graphics show it. It still remains a very complex yet fun game. There’s plenty of mass transit available, you can set up ordinances, and for the most part, it was well developed, and is a solid game. Solid that is, until your cities get large and the entire traffic/mass transit system breaks down. Custom addons/mods are the saving grace for SC4, but there are just too many things to fault with the game for them to fix them all. In some cases, an addon will make the game playable at the expense of removing all sense of realism/challenge in a certain area. You can get a fairly large well-functioning city without addons, but I guarantee you it won’t look anything like a real city.
Cities XL on the other hand grants a much better/smoother experience when growing into a large city. That said, the game isn’t quite as developed yet. It still seems like a beta (despite being released earlier this month) – there are bugs everywhere from one of the tutorials to various minor issues in the game itself. That said, the game is still being worked on, and will be constantly improved for the forseeable future. The graphics are a huge improvement. It both looks better and operates more smoothly than SC4. You’re not in a locked grid system – you can build roads that curve every which way if you’d like, and traffic is handled a lot better. However, Cities XL has more of a subscription/online/multi-player focus to it – we’ll get into more details about that later though.
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.