What are some alternatives?
When comparing PaintCube and Oculus Medium, 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.
MasterpieceVR - Powerful VR sculpting & painting tool
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Tilt Brush by Google - Paint in spaces around you with Virtual Reality
Mari - Mari is 3D painting without limits, combining the power and performance to handle even the most complex assets with artist-friendly 3D paint tools.
A-Painter - Paint in VR in your Browser