This is the boss mount:
This is the case mount:
The boss mount seems a more attractive and robust solution, if possibly a little heavier.
The boss mount has a simpler U-bracket which attaches the alternator, like can be seen here:
Apparently most Lycomings made since the 80's have a boss mount. Boss mount is said to be better, thus the change away from case mounting. The case mount brackets are less able to resist bending and crack easier / vibrate more.
