Of those two, definitely ketamine. Mushrooms don't ever let me STOP thinking, in fact that's pretty much all that happens. The hallucinations are largely ignored. Your experience may vary, though. Pretty much any psychedelic can take you to a place where you're just totally lost in the sensory experience in huge doses though, but in smaller ones most of them will have you thinking quite a lot. And the huge doses will have a strong impact on your psyche, even if you're not quite thinking about things in the traditional sense. Psychedelics are generally very stimulating. Most hallucinogens in general are, actually, at least mentally. I'd say that your closest bet would probably be dissociatives.