The 12mm manifold insulator gives around a 10% increase in low down torque, while the 25mm item gives around a 13% increase over a broader range, as shown in the link below for the 12mm item.
http://i305.photobucket.com/albums/nn210/mace-engineering-group/Automotive%20Products/Manifold%20Insulators/ALLOYTEC_DYNO_mace.jpg

In regards to which insulator is better, that would depend on your driving style. For argument's sake if the majority of your driving took place between the low to mid RPM range, then I would recommend our 25mm manifold insulator. If the majority of your driving took place in the mid to high RPM range, then I would stick with the 12mm manifold insulator instead.