said by Mr Matt:
contractual prices both carriers charge are basically the same.
Because it's not.
The base plans might appear to be the same but when you factor in the differences between them (rollover for AT&T, friends and family for Verizon, mobile to mobile for each carrier) the actual per-minute cost you wind up paying will vary depending on your needs and usage.
Nor do they charge the same for data since the last time I checked Verizon still has unlimited plans for smartphones and AT&T doesn't.