[edit] I get the feeling that you already have a favorite, but you're trying to talk yourself out of it?
____________
Now, on to the compare and contrast!

I like the one with the flowery hills. It makes you want to go there.
Your alien happy land had a beautiful purple atmospheric effect on the alien world. Was that done with overlapping shapes of different opacity? That technique might work here to give depth to the sky without using the gradient tool.
I've been looking at Futurama as a guide for my backgrounds and I notice that they maintain a cartoon-y look by using gradients here and there in a subtle way so you can't discern a difference from the non-gradient elements unless you're looking for it. But it's just enough so that the occasional dramatic gradient and the 3-d effects don't look out of place. But that's a pretty different style and might not be relevant here...
