At my previous job, we had Fastly as a potential new CDN provider set up against our existing CDN provider and two other new potentials. After a few rounds of calls for bids, Fastly won out.
Based on my experience with the other providers they were also, by a large margin, the most modern - it felt like moving from a 2008 integration to a modern, fully RESTful API with great documentation and decent UI.
This is all anecdotal, but they did combine a great technical platform with great support. If transit prices are the same or similar for all providers in that size category, they have to fight on features and support instead.