Well, there is some leeway in the voltage supplied through the mains. UK stuff can take 230-250. I bought a 100v synth a little while back and couldn't find a 100v converter here in the UK, so got a 240 to 110 instead. Absolutely fine. No problems at all.
The thing is, not only is there fluctuation in the mains supply, the transformer of the synth's internal supply won't put out a significantly different voltage to the regulators than it normally would. Standard 78/79 Regulators can handle about 30v typically, and will still put out their nominal 15v or whatever. Not all power supplies will use these regs, but the MS-20 does.
On the other hand, the MS taps off a nominal 18v from the diode rectifier before the regs - so if the transformer is putting out slightly more, the voltage that goes to the oscillator circuits will be more, which may affect their performance.
If you're handy and safe with electronics, you could try measuring the voltage applied to the regs, but this could be potentially very dangerous if you do not know that you are doing, so this is NOT a recommended approach unless you are an expert with such things. Mains voltages can be fatal.
Maybe try the other converter and see. I doubt you'll harm the synth, it might just need fine tuning to work properly.
Bear in mind I may be horribly, expensively wrong on this