Sprint will almost always be the cheapest for anyone, especially when considering data. Not to mention they have the fastest speeds (WiMAX with sprint tops out at 128Mbps down/56Mbps up vs Verizon's new LTE topping out at 100Mbps down/50Mbps up and T-Mobile's measely HSPA+ 4G doing 56Mbps down/22Mbps up). Not that you will ever see any of those speeds. The only benefit is that apparently Verizon's LTE is easier to upgrade in the future and (after said upgrade) would be 1Gbps down/100Mbps up.
But back to the current topic:
Price Comparison (Unlimited Text, Smartphone Data, and 1400-1500 mins for 2 lines):
Sprint: $130 for unlimited text/data 1500mins
- texting included as part of plan
- fully unlimited data, additional 2*$10=$20 if both phones are 4G such as Evo or Epic
Verizon: $120+(2*$30)=$180 for unlimited text/data 1400mins
- texting included as part of plan
- fully unlimited data
AT&T: $90+$30+(2*$25)=$170 for unlimited text/ 2GB data 1400mins
- $30 family texting plan
- 2 x $25 2GB data plans on both phones
Can't get a better comparison than that!