I'm going for the G4 and I would recommend the same for you. Not only does it STILL offer a removable battery that you can interchange between charges or replace when needed. But also still has expandable memory up to 2TB support. 2 things the Galaxy S6 is apparently lacking with its all glass back (one more thing to break), and as Samsung has decided to pull an Apple by doing away with expandable storage.. you also benefit from a more professional camera in the back. (At least on paper).
But my main driving point to getting the G4 is the fact that it doesn't rely on a physical home button or those annoying capacitive buttons next to it. I find those physical buttons hindering in most cases especially in the rare scenario that it malfunctions or stops functioning completely. (If iPhones are any indication after repeated use.) And as for the capacitive ones, well they're just downright annoying especially when they don't light up. I much prefer the onscreen navigation controls being that they adapt not only to orientation but also when extra menu options are available it makes the appropriate buttons visible. This is of course obtainable on the M9 but given I have an M8 as well, HTC's newest phone hasn't impressed me enough to consider they're new flagship and all of its same features that I have now, such as the same 1080p LCD panel, aside from the new camera which is unsurprisingly lackluster.
The only real luxury that you're losing with the G4 is its build materials. Not being made of glass nor aluminium so it doesn't come with the durable feel experience that the HTC and S6 give you. But of course the leather option is open to you if the plastic backs don't appeal. However these are luxuries you probably won't miss much, given all these phones will be in cases regardless.
As for performance the Hexa-core 808 has more than enough power to stand up to the M9s octa-core 810. And you'll see literally no difference in day to day performance between the two. In fact you may even see better battery life on the G4. And with 3GB of RAM you cant go wrong.
Unfortunately no quickcharge for the G4 despite the chipset supporting it. But it will support Qi wireless charging with the appropriate rear case.
Overall - just get the G4 unless you want to get an M8 lookalike. Otherwise if you dont mind physical/capacitive buttons along with some other neat sensors and capabilities and no removable battery or expandable storage. Get the Galaxy S6.