The 645MG has 133 oz of torque while the 5645MG has 168oz. on a 6volt setup. They are both metal gear and dual ball bearring. They are also both 3 pole motors. The 5645MG is slight quicker at ..18 second while the 645MG is rated at .20 seconds. (Again, under 6v config)
The digitals do not have a centerring pod to go bad! This is a GREAT thing in the servo world. And if you have an HRS RX you will get even faster response and added functionality. Direct Servo Control is also more precise.
I use the 645MG for Throttle/Break while using the 5645MG Digital for all steering applications. I run 6volt paks on all my rides.
They both work VERY well.