There is a significant difference in network coverage and bandwidth speeds. Also, until recently Sprint device selection severely lagged both AT&T and Verizon as the iPhone and many top Android models were missing.

There are many factors for choosing your carrier outside of price.

Sprint has almost no LTE coverage currently. They bet heavily on WiMax, which flopped. They're working to rapidly fix that, but they're way behind for now.

Coverage. Especially in the west.

Sprint users get free access to Verizon's network until 2016.

I know they have a 3G agreement, but do they use Verizon's LTE network too? That makes a huge difference.

The agreement is technology agnostic. I had to escalate up a few times and after some persistance, got a call back from someone who was engineering level.

Sprint roams on VZ's network.