Well you only have two options built-in - use the optical out built into the headphone socket or use USB. There are probably minor advantages to using the optical out, which should not be beset by the possibility of electrical interference from the Mac's power supply, but if you want to use higher-res audio (like 24 bit) then you should use the optical out anyway (though getting 24 bit out of it is a bit of a faff, since it doesn't recognise the bit-rate of the source material, and you have to change settings to get it).