If you are using headphones I don't think you'll see much of a difference. There are some custom ROMs for the Evo that have Beats, but obviously that won't be an option on the iPhone. If you are using line out (not the headphone jack) and amplifying the sound through your car or home stereo, the adapters for the iPhone are easy to come by. I would assume that you would need an MHL adapter to get a line level signal from the Evo.
I'm still hoping for usb otg or even better host mode support for adding an external dac/amp such as the Fios E11 or similar. This is definitely the best sounding phone that I have ever owned. It's so bad that I just purchased a 64 gb micro sd card to hold all of my media. I've officially retired the Cowen because of this phone.