Right now I have the evo 3d. Support for it was really good, actually still 2 or 3 developers working with it, so I'm running kitkat on it right now, which has kept me from feeling the need to upgrade right away.
Anyway, like a lot of other people I'm deciding between the m8 and s5, I might wait a little longer just to see what LG drops. I'm not too familiar with how developer friendly the Galaxy line of phones are, specifically the Sprint models. I've read varying opinions from there's little development outside of some TW roms to development is strong.
Developer support is a big deciding factor for me since it can really allow you to prolong the life of your phone, hence why I'm still using my evo 3d, though it's falling apart now and showing its age so its time to finally bite the bullet and upgrade.
Not asking anyone to predict the future on how dev support will be for the s5 but just looking for input on how it's been for the previous Galaxy phones just so I have some idea. Talking TW roms as well as AOSP roms.