Hi H,
I can understand you perfectly,
it's just tools and you want to use the one that best suit you
at a spefic action.
I think you only going to loose your time explaning that
a diferent program does not change the sound etc.
Can you keep both?
Don't put 10 years in the can.
Keep Protools for those you want,
and tell them that Logic it's better for preprodution ;o)
and use it when the client will not boss you around.
Another thing you may try is different rates,
Higher for Protools! :o)
Kindly Regards,
Jorge