Differences between the Original Spektrum Systems (grey cases) and the Pro Series Equipment:
When speaking of Pro Series Equipment, I am talking about the Pro Series modules with the smoked cases, SR3001 receiver and the SR3500 Micro receiver.
The SR3001 and SR3500 receivers feature a conformal coating on the circuit boards. This aids in making them fuel proof and water resistant
The Pro Series equipment features reduced latency
The SR3500 features a wider operating voltage range, eliminating many BEC issues that would have required use of the SPM1600 Voltage Protector
The Pro Series Equipment all feature smoked translucent cases while the Original Series gear has solid grey cases.
The Pro Series equipment has updated firmware to prevent ESD shutdown issues. Original SR3000 receivers that were purchased after May of 2006 have this firmware as well.
The SR3000 and SR3001 are both compatible with the Spektrum Telemetry Modules, the SR3500 is not.