I was stuck with the exact same dilemma on if I should be going with Paho or Mosquitto c++ client library for an embedded linux device. First, I tried a minimal application with PAHO as It supported every feature that I needed while the cpp-library-wrapper did not. So, my opinion of the main project branch paho.mqtt.cpp is that it has not been maintained too well, like say, they lack these two important features:
- set all connect options like mqtt-version, etc
These are not offered through the mainBranch cpp-lib in PAHO. I use libmosquitto with libmosquittopp wrapper now and as you can see, my decision leans towards the cpp-library only. But, just like you, I'm still curious about more differences between the underlying C-libraries.