Photon scales to 1000's of concurrent connections... at a price, which is pretty reasonable considering the hardware server you may need to get same.
They manage everything.
The plugin has everything you will need. and they offer tons of documentation so you can write your own.
Do it yourself vs 3rd party.
If you are super savvy and are a wizz with servers etc then Socket.IO plugin might be better.
But, if you would rather run a business, build a game and not spend all your time fine tuning a server etc, then photon is better choice.