You can't go wrong with either one. But since you mentioned that you will be using this with your personal fleet of vehicles with big panels and curved areas, I would personally go with Rupes 21. This way your able to use the 6" backing plate and have the option to use a 5" backing plate as well.
It challenges me to use all my skills and knowledge when it comes to detailing a vehicle. But in the end, its my customers satisfaction. Seeing them amaze and happy is priceless.