I tried both for a week (each). I wouldn’t say the 12.9 is uncomfortable; it’s more like “clumsy”. The difference in portability and size / weight can be summed up as:
- The 12.9 is a device I’ll bring with me when I know I’ll use it. If I‘m not sure, then I’ll likely leave it at home / on the desk connectors to the Magic Keyboard.
- The 11 is a device I’ll bring with me when I think I might use it. If I’m not sure, I’ll bring it anyway just for the hell of it.
Similar except I always bring the 11" with me (fits inside my purses).
The 12.9", I need to carry a separate laptop bag so I don't bring it everywhere.