For me, taking practice tests was a matter of what was closest to the real test rather than what was more difficult. I don't have much experience with TPR, but I did use a lot of the AAMC practice tests. They were a fairly accurate gauge of what to expect come exam day. Their difficulty level was similar, I felt, to the real thing. That was really nice because I could better gauge the amount of time I normally took on a passage.
That being said, remember that difficulty is relative. Your final MCAT score is scaled. On really hard exams, you could miss more and get a higher score. I'd rather take the more realistic (somewhat easier) practice exams and learn not to make particular key mistakes, which are more costly in terms of your final score.