I've only been playing Cities XL a day, compared to playing Sim City 4 since 2003, but I wouldn't say I prefer one over the other. Cities XL has a lot more 'Please build this now. Why aren't you building this now? If you don't build this now, everyone's going to move out and no-one will have jobs', which is both useful and annoying, but the graphics are beautiful (you can watch a car drive from its house, park at a nearby store, see the people get out of the car, go into the store, come back out, get back in the car, then drive off again), the roads are much more customizable, you can have weirdly shaped farms, there are both ploppable and zonable building (I really like the European 'medieval' style building), you have an entire planet to play on, and the buildings aren't as repetitive. You still get several of the same type of building, but, especially in commercial zones, they're not all together in a row. You won't get 600 Wren Insurance in the same block in this game. At least, I hope not. I'm nowhere near skyscrapers yet. I also like that there's a built-in cheat mode. You just click the Cities XL menu, go to the wrench, and then select 'cheat', and you can put your money back up to 500,000, unlock buildings, that sort of thing. Other things I don't like: it's not as customizable. There's less of a 'sandbox' feel to it, in that you only have certain buildings available until your population grows enough. You can't just go in, place a road, place some residential, commercial, agricultural, industrial, police, school, fire department, etc; then watch it grow. But maybe it's more realistic that you can't build your first school until the population is over 5000. I mean, in SC4 the education advisor would always pop up and whine if you had a school with too few pupils. While there is an entire planet, and many cities available to work with, you can't pick where they are (they are pre-determined by the game), you can't load other 'planets', and, as far as I can tell so far, you don't seem to be able to connect different cities via a complex transport system, like you can with the regions in SC4.
So which one to go for, I don't know. Both have good and bad sides. Sim City 4 is more customizable, and, not being full 3D, runs faster on modern computers, but the graphics aren't as good, obviously, and there's the whole commuting problem (especially before NAM).
I think SC4 has the advantage overall, to be honest, but Cities XL is an excellent game.
