The recommendation is always to take the official GMAT Prep (at least for now) to get your score prediction confidence level up. However, if your scores are consistent, we have heard on the TOTAL/OVERALL level, 70% of 200 or so users we surveyed in May said that their final GMAT score was within 50 points of the
GMAT Club test. Within those 70%, about 50% said it was within 30 points or something along those lines (sorry, going from memory here) but this is not critical because you don't know which group you are in anyway
So GMAT Prep would be the best for you. In terms of more difficulty questions,
don't treat that more difficult questions = lower score. More difficulty questions means more challenge but the scoring tends to be close. The system adjusts for the mix of questions and we see often on the Quant it takes just 2 mistakes to get Q80 or Q82 for example on the Real Exam while in GMAT Club you can make 4-5 and still get a high score because of higher challenge. The test does not depend on the number of incorrect answers but your skill level.
On the Real GMAT, the accuracy has to be really high to get a high Q score. So keep an eye out for those careless mistakes.