No, you will not notice a difference between the Monster priced cable and the basic Y cable. Be wary of the 'upsell' - YOU can make the decision despite what they want to sell.
MP3 is of course a digital format and you will sending the bits over the air. The Airport Express will then decode it, convert to analog, and send it out the analog jacks to your receiver. The act of sending the digital information wirelessly will not degrade the signal in any way but you may run the risk of dropouts IF you have any interference from other RF devices (like say a wirelss phone, cell phone, microwave oven).