I recently noticed that Waves doesn’t support the Audio Unit versions of their plugins in Ableton Live. So any Wave user should choose the VST versions instead.
At first I felt that Audio Units, being a specific Apple format, should be preferred over VST, but I was wrong.
I imagine that the reasoning behind using an AU in Live is that people think “well, I’m on an Apple. The AU must be better!!!” This is false. The AU is never better. Ever. You might like the preset mechanism more, but as far as Live’s presets go, this is a transparent experience to the user, and thus moot. The only reason you’d have for using an AU in Live is if you want to make presets there and then use them in Logic or DP, or vice-versa. If you don’t do this, use the VST.
In most cases AU is using the VST code with an extra layer of code on top to make it act like an AU.
Conclusion: It is safe to say that VST is the preferred format for any Ableton Live user, even when you are on a Mac.