After doing some basic research, I have noticed that except for a few of the discount airlines--Ryanair, Wizz, Easyjet, etc--most European airlines now limit the personal item to 40 x 30 x 15 cm. (Most US airlines allow larger ones.)
I am buying a new bag this week to use as a personal item. I have narrowed it down to two bags--both exactly the same except for size. (Same manufacturer, same model.)
The first is a backpack measuring 40 x 30 x 20 cm. It has four external cinch straps so if not packed full can be cinched down to around 15 cm. (For the non-metric folk, 5 cm is about 2 inches.)
The other backpack is 40 x 30 x 15 cm but can be expanded to 20 cm. It does weigh a bit more than its larger sibling due to the expansion feature.
The price is basically the same. Which would you buy?