The biggest factor is really your environment. When I took the PSAT as a junior, I had already taken it as a sophomore and, of course, I was taking it at my school. When I took the SAT for the first time I didn't know much about it, and I took it at the CSU with this crazy proctor. Basically, the test completely freaked me out (and I never had major test anxiety before the SAT) and I didn't do so hot the first time.
Also, in my opinion, the Critical Reading is harder. I got a 78 on the PSAT, but my highest SAT CR is 710.