Some people argue that with a 3r2.5, you have the flexibility to button the 3rd button or not. Some people like the longer roll in a 3r2, others like the symmetry of seeing the top and bottom button. The roll of a 3r2.5 is also often more voluminous while 3r2 are often flatter.
November day, much like this one on the East coast. Tweed jacket, cap and scarf is all you need (if it weren't raining) and the whole is made more comfortable for the walk home from the pub when the tree buttons are done and the collar is flipped up so the lapels close over the chest.
I don't recall ever seeing the top buttoned closed and finding it to be flattering. In fact, this has always stuck out to me as an unfortunate look. If it were cold outside I'd rather wear a waistcoat and stick with closing the 2nd button.