T-Mobile shows that the Galaxy S5 on their network will get the Marshmallow upgraded and it is rumored to be released sometime in May. Whether or not the update actually comes out in May or is staggered between the carriers is uncertain.
In my opinion, the Galaxy S5 still performs remarkably well. Unless you actually need a new smartphone, it should suffice for most casual users until the Galaxy S8 comes out. I bought a new T-Mobile variant last week to get Wi-Fi calling and improved network support compared to the unlocked Lumia 640 XL I was using. The 640 XL and the Galaxy S5 have more in common than most think, given that they are both running what could arguably be their last official software release. Windows 10 Mobile has been delayed many times over and most people have come to terms with Android 6.0 not coming to the Galaxy S5 because of reasons.
For argument's sake, I have used Marshmallow on a S5 and Windows 10 Mobile on a 640 XL. While the builds for each are community supported and not final, stable builds meant for daily use it gave me a good idea of what I could expect down the road. I'm more excited about Marshmallow on the S5 than I am for Windows 10 Mobile on the 640 XL, I really see Marshmallow extending the life of the S5 for another year. I see Windows 10 Mobile unifying the desktop and mobile experiences, but the the lack of applications and under-powered hardware will keep the 640 XL best suited for Windows 8.1.