I am currently trying to develop some vr apps for Google Cardboard, Rift etc...
I guess at the end of the day it is clear that BabylonJS looks more developed than A-Frame, but A-Frame is built by Mozilla, which means:
1). It will likely stay around for some time (though it is built on top of ThreeJS, not sure what happens when that stops being developed).
2). Will stay open and remain with an open license
3). Will follow new specs as they are released, and contribute to them
4). Possibly better experience for VR since that is there focus
I do like that BabylonJS seems more powerful though. Any thoughts, pros-cons?