What stylus is good for taking notes and writing equation
If you want something that is significantly better than the Adobe Ink, you'll need to go for an iPad Pro with an Apple Pencil. The combination of higher display refresh rates on the iPad Pro and the higher rate of stylus updates from the pen itself - it's really much, much better.

I use the Apple Pencil to write mathematics and draw diagrams/graphs, and it works well for me. However YMMV so be sure to go to a local Apple vendor and try it out before buying.