Hello,
Basically QMI (Qualcomm MSM Interface) a proprietary protocol that offers more accessible and faster connections to mobile networks Where PPP (Point-to-Point Protocol) is a traditional communication protocol that is widely used and most flexible in most devices.
To sum the difference between QMI and PPP. QMI provides much faster connections while PPP is an old protocol that is mostly compatible with devices.
I hope it helps.
Regards,
Mellow