[sciurius]

The QUAD is much bigger (wider) than the DUO so the added functionality for the two pedals better be worth it. You need to be careful to hit the desired pedal. I have a (guitar)effects box with 4 'pedals' (single push type, just like the DUO) and I always have to look first before hitting the right pedal. This may or may not be acceptable depending on the type of performance/instrument/etc.
Enter/exit annotations mode doesn't seem to be handy to me. After entering annotations mode you need to touch the screen to add anotations anyway, so entering annotations mode with a three-finger touch is easy and straightforward enough.
From my personal experiences, while in performance mode, I most often touch the screen to start/stop the metronome and to exit performance mode. Would I assign these to pedals? I don't think so.

BTW: When buying an AT DUO, make sure it's a BT-106, not BT-105.