Aside of redundancy of transformer primaries when using a step-up, there is no electrical or sonic difference between the tow I can discern.
As long as the the step-up transformer can deliver the current required by the mic without overheating, it does the same thing as the built-in input transformer in your power supply: it taps into a section rather than the full coil of the primary to link the 110VAC * line voltage to the two secondaries of that transformer.
I would do the line voltage matching "on board". That's much simpler and less prone to the vagaries and unknowns of the third-party step-up box you would need to add.
* Keep in mind that you will have to do careful output voltage adjustments to match the tube(s) n your SM2, once done with your line voltage issue: These old supplies were designed to run on nominal 110VAC rather than the current 120. AC701 tubes don't like a heater voltage even a tenth of a volt above 4.0VDC.
