How to pic the best CDN available for hosting locally

I have few questions on CDN.I am hosting locally and I want to pick the best CDN available. How to choose the most efficient one for my purpose? What is 3G latency concept on CDN? How can I manage performance fluctuation with a CDN and can it provide guarantee of 24/7 availability of my site services? Can it help me render some heavy pages faster? How is SPDY related to CDN and what industries use SPDY?
