AU or VST ?

Depends on the platform the DAW was coded for first. If it's a Windows host that was ported to Mac, use VST. If its an original Mac program, use AU. Developers tend to handle their original platform better than a port. 

For the Cognitone Audio Engine, use AU plug-ins where possible. 

Never use VST and AU drones in a DAW at the same time.

If it works, there is no reason not to do it. The above rule of thumb is only when you are in doubt.