I love both Pono and Opio, therefore I really like your question and that you narrowed down your choices to two brands which I'd consider to be the best value available.
I adore pretty much everything that Pono is offering, considering all the wood options and different choices or upgrades like slotted headstock, cutaway, radius fretboard, truss rod, gloss or satin finish, very clean build, and now a side sound port and even a bevelled armrest on their higher line of instruments... In terms of what you get for your money, I would say Pono has the edge. When it comes to the sound, however, I have to admit that I prefer the Opio (mine is from the Sapele line, before they switched to Acacia, see my review
here). It's not world's apart, but definitely louder and fuller, matching some of the best custom ukes I have owned.
I wonder how many people have actually measured the neck thickness? I've done it on all of my ukes and came to conclude that the neck profile on the Opio tenor I have is pretty similar to that of Pono. Both necks are definitely not thin, but perfectly round which feels very comfortable to me.
Looking forward to hearing what you decide. Can't really go wrong with either one.