Player.IO versus the Photon Server
Player.IO does a number of things differently from Photon Server. One of the biggest differences is that Player.IO is cloud-hosted. That is, your game code runs on a cluster of shared servers (whereas Photon requires you to host your own server), and it is also possible to rent dedicated clusters in this cloud. Additionally, Player.IO is room-based. Upon connecting to Player.IO, you specify a room to connect to, and players are segregated into different rooms. For instance, in an MMO you might have different "rooms" for regions in the world. Without a dedicated cluster, rooms are limited to 45 players. If you rent a dedicated cluster, there is no limit.
One final, big difference in Player.IO: as Player.IO was originally designed for Flash applications, and Flash does not support UDP, Player.IO supports TCP only. Additionally, Player.IO has a strict limit on message processing, and too many messages can result in disconnects.
Other than that, most...