16" wheels fold smaller and stronger, but require suspension.
Some folks think fat tires afford adequate suspension, but there is a big weight loss there. There aren't many decent fat tires in this size, either, though the thorn resistant Greenspeed Scorchers might be an option. (The non-TR tires go flat if you look at them funny.)
I prefer the 349 size, but only on a suspended bike, such as the Birdy or Moulton. There is debate about whether efficiency differs by wheel size (given the same tire and road condition), but there is little doubt that an unsuspended wheel in this size will give a harsh ride.