A left field choice might be the XFactory X6-S conversion kit. Only cost efficient if you have B44/X6cubed cars already but you said, cost no object
I have one and am really enjoying it, I get better results than I did with a regular 2WD and actually prefer driving it to my 4WD and I feel faster with it. I am not a great driver, just in my 2nd year but it is a very easy car to drive which is probably flattering me.
That said there are several KF's at our club which are all doing well so if you want a KF/XRay preference, I would go KF.