What are some alternatives?
When comparing PaintCube and Mari, you can also consider the following products
Substance Painter - Substance Painter is a 3D painting software allowing you to texture, render and share your work.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
3D-Coat - 3D-Coat is the one application that has all the tools you need to take your 3D idea from a block of digital clay all the way to a production ready, fully textured organic or hard surface model.
Armor Paint - Armor paint is a standalone software for 3d texture painting in PBR.
ZBrush - ZBrush is a digital sculpting and painting software solution.
Sculptura - 3D sculpting for everyone