It seems to me it would be very difficult for the average person to formulate an unbiased opinion... How many of us have actually encountered both? And further... the table itself, age of the rails, current condition, quality of the balls... even the environment... all have subtle and not so subtle effects on play.
With so few quality tables in my area... I personally don't think I have ever had the opportunity to play on Granito M cloth. The heated Gabriels tables I do play on are covered in Simonis and they play fantastic. The smaller, non-heated tables surely have a considerably lower grade cloth and play like slogging though quicksand in comparison.
Perhaps if in a magical world we could set up two tables from the same manufacturer... newly installed and covered with each type of cloth... sitting side-by-side in the same environment. That would be the best way to have someone truly compare... if that person had the skills and sensitivity to even detect a difference!