I picked up the free copy of WB 3.6 that was included with 3D World Mag. This is a great deal, as WB (certainly WB 4) is pretty much on par, or better than Vue Infinite.

You can do pretty good trees in Carrara, but the stock ones really don't demonstrate Carraras ability. I've created (modified) some C5Pro pines that look better close-up than Vue trees. One thing that is excellent is adding the translucency channel to C5 trees -- makes for some fantastic lighting effects, especially with dusk or dawn scenes.

Don't forget to checkout Onyx Computing for plant creation. They have plant/tree creation software that is pretty much on par with XFrog, but (I understand) is much easier to use. You can also create wind simulation with their plants.

http://www.onyxtree.com/