I have Sprint and my 4g (in Salt Lake City) is horrible compared to my friends Verizon... of course he pays like $60 more than me, but still... doesn't even seem like 4g on my iPhone, more like 2.5g.
Nothing compares to Verizon's 4G LTE network at this point. It is in something like 200 markets already and you'll get speeds of 4-5Mbps on a slow day.
The iPhone runs on 3G on both Verizon and Sprint. That said, Verizon's 4G LTE has been shown time and time again to absolutely blow Sprint's 4G out of the water.