Some company uses 'T', some uses 'R'. But the important figure is the number of 'kv' and the 'wattage' of the motor. http://www.teamnovak.com/products/brushless/motor_spec_chart.htm
For the same line of motors, lower # of 'R' is faster. (eg: Novak Velociti 4.5R > Velociti 5.5R)