Another important consideration is toughness and with a bike that folds and stows this becomes extra critical as they can get banged around in transport.
Again, the 20 meets this requirement for me and they can stand up to terrain and loads that would make other bikes fold up...
I started with 1 Phillips 20 and now we have four of them as my daughter and wife have one, and I have two, and also have a Dawes and a Moulton (non folding) that I am building up.
If I was a multi modal traveler and needed to bring my bike on buses and trains a Brompton would probably top the list but since I usually bus and there are racks, my 20's fit up there like any other bike.