What timing pulley and belt you are using for your 3d printer?
As RobotDigg's experience, T2.5 pulley are still prevailing in South America and Asia. Most of the wholesalers and 3d printer accessory stores are keeping T2.5 pulley and T2.5 belt in Stock, it's hard to find a source of GT2 pulley and belt, so the situation stays.
MXL timing pulley and belt, we have no experience but RobotDigg know it's Europe standard and you could find nextdayreprap uk are of this kind supplier. In China, we take MXL/XL and T2.5/T5 are similar tooth profile with same efforts.
RobotDigg recommend you the option of GT2 pulley and gt2 belt for your 3d printer, as you may hear that gt2 pulley and belt give you anti-backlash drive. And we supply you the good quality and competitive price gt2 pulley and gt2 belt.
S2M is similar grade of GT2 from Misimi, a Japan origin company. In fact, S2M is not arc tooth/round tooth or circular tooth belt.
Misumi has given the following description to kinds of Timing Belt:
MXL/XL/L/H Type of Belt, Regular Torque, general purpose timing pulleys suitable for torque transmission and light load conveyance.
S2M and more, High Torque, timing pulleys for high torque transmission.
GT2/2GT and more, High Accuracy Positioning, timing pulley with small backlash, suitable for positioning.
T2.5/T5, Light load conveyance regular torque, trapezodial toothed timing pulleys suitable for conveyance, also usable for transmission.
So it's clear, reprapstrap, T2.5 and MXL is good and other reprap models RobotDigg suggest GT2 pulley and belt to the most.