Getting the iphone 4 for verizon vs at&t vs buying it from Apple?

My phone has gone to shits and Im really interusted in buying the iphone4.But of course I want the best deal for it. I was curious as to weather buying the iphone 4 from verizon&staying on the plan im on would be better. Or switching onto my friends plan and being added on with buying the iphone 4. Or even would it be cheaper buying it from apple itself?