The functionality of the Mac version of Mind Manager definitely lags behind that of the PC version. However, the files do appear to be portable across platforms. There is also a reader program available for both platforms.
In my experience, the basic mapping functionality works fine on both platforms.Where you're likely to run into problems is in the interfaces with programs other than Mind Manager. The Results Manager add-in from Gyronix only exists for the PC. The PC has much more complete export/import options for connecting to Microsoft Office. Stuff like that.
I'd suggest checking out the free trial on both platforms. That will give you a better idea of how the differences affect what you personally are trying to do.