Medentry should be slightly harder.
The ACER practice questions are purposely much easier than the actual UMAT exam. People doing the ACER practice exams think UMAT is easy, have a false sense of security, hence they pay 50 bucks to join and then get killed during exam.
I quite strongly disagree with this, having done the test in 2009.
Yes, medentry papers tend to be harder, but that's not the only thing that matters. Medentry's certainly were harder in my year than ACER's, but the two papers felt so different that the practice did not seem to be very meaningful (at least for me) for factors other than that it makes you get used to reading things faster and section 3.
The different 'feelings' that the papers gave were especially prominent in section 1. It was definitely those official practice questions that represented the real UMAT questions better than all others, although yes, they were easier than the real ones (but not THAT much difference imo).