Mudbox and Mari are both really nice. But for realtime PBR work or procedural painting right out of the box, Substance PAinter hands down. Mari is a hog for resources and Mudbox has it's niche, but real time texturing isn't what it was designed for, it was built to compete more with Zbrush as far as digital sculpting goes. Mudbox lacks a lot of features, has limited brush sets, and doesn't do PBR painting without some updates and tools to help it along. That said, if Substance craps out on your machine, Mudbox is probably the way to do, plus it's an autodesk product which means you can technically get it free. 😀