kray is amazing..
I've tried a fast test using maxwell and kray with the same scene..
my objective was to keep very small changes between the two settings...
here my rules...
illumination:
only white luminous polygons behind the windows, in maxwell they are emitters, in kray i've turned luminosity to 500%
materials:
in maxwell I've used wizard for transparent and reflective materials, all the rest (walls and ceilings) was basic diffuse white material
in kray I've played with gradients and reflection blur
global settings:
on maxwell, just click "render"

the results (NOTE: I've used different floor texture so color has changed...)
maxwell: 10 hours rendering.. a little bit of noise.. smooth image.. perfect refraction.. very realistic reflections..
kray: 30 minutes rendering!, sharp image, nice reflection.. BUT very non-realistic refraction..
so... I think the winner is kray.. but..
here's my question... how to improve glasses?..
the air polys tweak works also in kray?
thanks to all.. and sorry for my english..