I prefer to use VST over AU whenever possible.
One reason is because (I believe) VST plugins are tested in more hosts than are AUs, the latter really only relevant for Apple’s own product (Logic, Garageband, MS). So everyone HAS to provide their plugins in AU format but most likely only test them with Apple hosts. VST plugins get tested on a much wider selection of hosts so are more likely to be reliable.
That said, watch out for Kontakt 7 — apparently it is quite buggy in general. I am personally staying with Kontakt 6
Just my two dents.