I have both the pono and the digital Sony walkman and use them travelling with the top of the line AKG earbuds. Sony wins hands down in my opinion -- device has more sound configuration/EQ options/hi def settings (yielding better sound), better physical device controls and much easier to use file loading and management. Pono has some configurability but nothing about the pono software or the pono store (very beta) is intuitive. I was able to tweak the Sony with no instructions to yield great sound for travel. Haven't found a case for the Sony yet (Pono includes one) but the Sony at roughly the size of a fold open gum pack easily fits in my shirt pocket while the pono looks like I'm packing a steely dan (the pono's wedge shape and size though does slide well down the airplane aisle on takeoff if not strapped in - ask me how I know that . I got my Sony for $299 online.
To me both sounded better unconfigured out of the box than my iPhone6 but the pono is just a maddening device to load, adjust and use while the Sony sounds and behaves like my iPhone only way better. Hope this helps.
