You'll need some serious horsepower for this, but Mari does support 32k textures. I believe the only other app that does is Mudbox, but that's pretty much Autodesk abandonware.
It's been a bit since, but I've used both Mari and Painter in the past. When it comes to painting I prefer Mari's projection painting system over Painter's traditional paint on model system. However Substance does have a lot of nice functionality to let you work very quickly, particularly when using Painter in conjunction with Designer. It's too difficult to make a proper comparison typing on a phone with one thumb. Mari just provides a different workflow and very deep level of control over things (designed for VFX first rather than games).
As for cost, yeah, Foundry stuff is not the cheapest. For individuals you're looking at just shy of $700USD/yr for Mari if you can handle up-front costs, saving two months, otherwise it takes you a bit over $800 when done monthly (annual plan paid monthly). This compared to the Substance Suite, primarily Painter, Designer, and Sampler, that's available for $50/m or $550/yr.