PaganArtist and replicand hit the nail on the head. Indeed, Carrara is like what Poser should be although posing characters remains easiest in Poser itself. Carrara is cool but ultimately Vue is superior for its advanced atmospherics and the degree of environmental control it grants the user. 

To ease the initial investment (and ensure that you want to take it further) the best entry/upgrade path is to buy Vue 6 Esprit first, wait for a sale (like the one going on right now) to upgrade to Pro Studio. After the "getting to know you" period you can evaluate whether or not it would be advantageous to upgrade to Infinite. Unless you're a 3D professional I wouldn't necessarily advocate diving into Infinite off the bat, although if you're tempted to do so, as stormchaser already said there's the free PLE for test-driving.

I wouldn't recommend buying the individual expansion modules for Vue 6 Esprit. You WILL come to want at least Botanica and Ecosystems, but you'll save money buy simply upgrading to Pro Studio, especially when it's on sale. e-on's sales are generous. If you play your cards right, you can save 50% on Vue upgrades.

 Although the ostensible subject is Poser compatibility, I sometimes create terrains in Carrara to use in Vue, just for some variation. That works beautifully! Ultimately, if you have both Carrara and Vue for use with Poser, you probably won't regret the freedom that having multiple rendering apps bestows.