It depends on a few things: which model megger and which model Baker (now SKF)?
From the most basic difference:
1. The 'megger' is actually and insulation to ground tester. It's purpose is to place a DC potential between the windings and ground. The result will be a measurement of leakage in milli, micro or pico amps that are converted over to MegOhms. This is a method of evaluating the condition between the conductors and ground of the insulation system. For detailed information on the use of the megger, either the instructions that came with it or IEEE Std 43-2000 "IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery." A megger is not a stress test of the windings and is normally performed before high voltage testing such as your Baker to reduce the risk of winding damage by the higher stress voltages of the surge tester.
2. The surge tester (or AWA?) evaluates the condition of the insulation between the conductors of the coils in the motor between turns, coils or phases, and can also detect ground faults and certain connection conditions. When properly applied, the surge tester will detect interturn insulation weakness in the insulation system. The associated IEEE standard is IEEE Std 522-2004 "IEEE Guide for Testing Turn Insulation of Form-Wound Stator Coils for Alternating-Current Electric Machines."
Both standards can be obtained through the IEEE Website: http://www.ieee.org
under the standards section.