Just tossing my two cents in here...
The voice quality of Sprint has been one of the company's strongest assets over the years.
Besides the Unlimited data, the voice quality is one of the reasons that I have continued to stick with Sprint.
I also agree that AT&T has crappy voice quality. My boss has an iPhone on AT&T, and frequently her calls drop or are garbled. Also, when I had a company owned Blackberry, it was on AT&T, and while the data seemed to be fine, the voice quality was crappy... I never had any dropped calls, but every call sounded tinny and some calls were garbled.