Yesterday I drove a 2014 Sorento SX, and I must say that I was very impressed. The refinement was especially impressive, compared to an RX350. (I am a previous RX350 owner.) One surprise: the third row seat in the Sorento is actually useable. It's difficult to get to, but once in there it would be tolerable for an adult for a short trip. Probably the most impressive feature is the nav screen. It's more user friendly than the RX's, and the readability of the screen is outstanding. I think the new Sorento is going to be a big success. Of course, questions about reliability and resale remain unanswered, The RX certainly has those bases covered.
Coincidentally, the Kia dealer is right next to a Lexus dealer, and they have the same owner. I think there will be some cross shopping going on.

 

Multiple brands under the same franchise is very common

 

The Sorento indeed has a better interior than the less-expensive and overly-plain Sportage, and is (probably) more comfortable and better-driving on the road. But, to my senses at least, the Sportage feels more solidly-built, overall, and the doors close with a more solid thunk. Of course I realize, though, that a lot more is involved in buying a vehicle than just how solid the doors feel.