I'm on Verizon and T-Mobile and I gave up waiting for the Z4V. First it was August, now it's September, and now I see rumors of the Z5 coming out soon! I hate what Sony does in the US (basically wait until a product is outdated to release a carrier-specific version). Anyway, now that my rant is over, to be honest I think they are on part sound quality-wise. As I mentioned earlier, I think the Z3+ (and Z4V for that matter, hopefully), has great treble extension and sounds overall very nice. I slightly prefer the warmer signature of the Note 5, but that's personal preference.
I have to ask, though: how much do you value the sound quality alone in the phone? The Note 5 blows the Sony out of the water in the categories of screen, camera, user interface, updates, support, processing power and overall snappiness. If I were choosing between those, I would choose the Note 5 without question. But since I have the Note 5 on T-Mobile, I am considering the Z4V on Verizon if it ever arrives. Hope that doesn't complicate your situation
-Collin-