I had a quick three way trial with AK Jr, Plenue 1, and ZX2 today.
SQ:
Plenue 1>ZX2>AK JR
Somewhat expected. Plenue 1 has a really rich signature(with headphone mode off, all eq setting off). I've never heard my Z5 play Norah Jones like it did. On the other hand, if you don't like warm signature, Plenue 1 might not be so great. ZX2, is also pretty good, though it's quite different from Plenue 1 too. It has it's own engine that alters the sound. Sound stage is absurd(good way), and separation is awesome. It has a capability of creating a sound scape like nothing else. It also smoothens out the treble with some sort of magic it performs. It also works really well with Z5. And my AK Jr. It's good. It can sound a bit edgy, but for half the price of the other two DAPs, I really can't complain. Besides, AK Jr. was perhaps the most analytical of all three. So if you like that sig, I don't see why you won't choose AK Jr. as your go-to DAP(unless you have something like AK380 of course)
Form Factor/Other things
AK Jr>P1=ZX2
AK Jr is the smallest and is as solidly built as the other two, making it the winner. However, all of them are built extremely well, although ZX2 was surprisingly very heavy. For a Sony DAP, it's really not what you would expect. Nevertheless, it also packs really good power.