between those 2 the VF SU6 would be the best option, the ram/CPU/Cam are exactly the same, but the VF has a bigger higher resolution screen, it has more storage, 16GB Vs 8GB and the sony has only about 1.7 available to apps thats pants these days. the sony does have the advantage of being waterproof and having a slightly more manageable screen size, id personally have the 5.5 VF over the 5 M4 though as i prefer bigger screen , The sony is getting upgraded to android 6 i doubt the VF ever will, though i doubt that's of bother to most people,
i have personally use a ZTE Blade S6 Plus for a few months the VF is a rebadged zTE Blade S6 Plus butthe VF has higher screen resolution. and tbff it wasn't a bad experience for the price they are pretty dam good .
hope this helps a bit
Ash