Looks like its mostly a price difference. The CU575 offers a color external display while the CU515 doesn't. Also, and i'm not 100% sure, I don't believe the CU515 offers the capacity of a 4GB microSD card. External MP3 buttons are not available on the 515 like 575. This is a mid-level phone, but if its anything like the CU500... WOW~!!!!! now that's a great phone! So I hope with this it only gets better!