Sprint's network is far from perfect, but it's less expensive, so pay less and get crappier service than Verizon. It is what it is.
As far as phones, I just did a pretty extensive search for a new phone, because I was due for an upgrade. I ended up going with the S6, because overall, in the videos I've watched, it always ends up on top. That being said, after having it for a few weeks, I've noticed that it had slowed down much more than my LG G2 did after having it for a couple years. I've heard that the S6 gets pretty glitchy, and that is what I'm seeing. It slows down substantially on at least a few occasions each day.
Based on my experience with both, LG's version of the Android OS is much more user friendly too. If the LG G4 would have been the same size as the G2 (instead of getting bigger), I would have probably stuck with LG.
I looked at The HTC One (M9) solely because of the front speakers, but honestly the sound quality improvement isn't nearly worth it when you take into account that the volume in speaker mode isn't nearly as loud as either the S6 or the G4. I had an HTC prior to my LG, and I really liked the Android OS of HTC probably the best. That's been about a year and a half ago though, so I'm sure things have changed a bit.
To me, it came down to being between the Samsung Galaxy S6, LG G4, and the HTC One (M9), because the size of the others was prohibitive...and I will not use an effing holster (it's not 1998). I eliminated the HTC One because the speakers weren't loud enough on speaker mode. I have a fair number of conference calls where I like to use speaker mode.
So...to me it then came down to the S6 and G4. Both are very good choices, but if I had to do it again, I might reconsider, given the fact that the LG version of Android OS is more user friendly than Samsung, battery life of S6 isn't as good as G4, and that my S6 has become slower than my G2 in 3 weeks of use.
I didn't even really look at the iPhone, because I've never been a fan of the Apple computer OS. It's always seemed antiquated to me. I hear the iPhone isn't nearly as customizable as Android either, but that may be up for debate.