AT&T is the only company to provide data and voice at the same time on iPhones. And it's the best choice for quality in many markets such as Boston.
One way I found I could speak out: I just downgraded my SMS from unlimited to pay to go ($0.20 per SMS). With iMessage, my texts are down to 30-50 a month so I'll save about $10 a month on average (down to $82/month). Before doing so just check your monthly usage beforehand to be sure your average is less than about 100 texts.