I have the RAZR Maxx HD and get simultaneous Voice/Data no matter where I am... 3g/4g, whatever. The new devices are built, essentially, expecting you're perpetually in a solid 4g area, getting simultaneous voice/data by way of VoLTE. I'm sure that will work for the vast majority of their customers. Traveling as much as I do through rural areas, that's not an option for me. Since AT&T has a different setup, I can get simultaneous V/D with any product on their network, not being bound to a 3-year old device to get it.
Here's where we had a good discussion on it already.