I did a bit of Photon and Photon appears to have more orientation towards client side multiplayer. Photon has a cloud server, very limited but easy to connect to. Unity's old default multiplayer, you had to set up your server, which could possibly lead to some issues if not handled correctly. (Eg. Players can't connect to your server unless you port forward or use Hamachi). Photon cloud felt laggier but was easy enough to connect with a single click. Note that Photon's documentation/support isn't as powerful in comparison to Unity. So whenever you need help, you'll really need to dig through Google/Youtube and Photon's website.

UNet although still in development, looks much better in the long run. It has/will have support for authoritative server which is useful to prevent/reduce the amount of cheaters for your online game. It also appears to have more options to how you want to design your multiplayer game; through Unity Cloud or your own way.

These are still early impressions, as I haven't gotten the most experience of all 3 network models (Old Unity Network, Photon, and UNet). Photon seems to be easy/fast to get into but less rewarding for demanding and advanced multiplayer games. UNet appears to be more flexible but more knowledge may be required to get a multiplayer game fully functional. Considering UNet will be an official part of Unity, there may be more tutorials and support for it.