I think this would depend on how far along you are in your preparation. If you think you need a lot of work, I'd recommend getting both and using the Kaplan Premier first, with the 800 later in your studies. If you are pretty well off, and you just want to get over the hump (maybe you're getting 680's consistently on practice exams and you want to bump it up to 700+), then go with 800. The questions are harder, and it's good practice.

There is definitely overlap. The Premier will have hard questions too, but it's just more of a complete overview. While 800 is geared specifically towards helping you answer hard questions better.