The GMAT Prep is probably the most reliable mock CAT out there.
But I can speak about the reliability of the GMAT Prep scores only when actual test conditions are simulated. Please take another GMAT Prep in full and update your scores (in full - along with the 2 essays, the Quant and the Verbal sections in one sitting only [10 minute breaks]).

Cheers!

itheenigma is correct. The GMATPrep is the most reliable in terms of overall score but the scaled score could certainly vary by a few points. A 50 on the GMATPrep test at home without the AWA, without the pressure, without the other people in the room, etc is easier to obtain as compared to the actual test.

Try this, take the FULL test including the AWA and the verbal. Do this at a location such as the public library where there are people moving around. Put the pressure on yourself. Tell yourself that only if you get a certain score will you sign up. Try to convince yourself it is the real test. See how you do with the distractions, the pressure, the full 3 1/2 hours plus of the actual test. It is likely that you can take it before December but this will tell you. If you take the whole thing your results should be pretty reliable.