plain or graph paper for case interviews better?

Whatever works best for you - your notes are non-evaluative (except in the context of communicating your structure) and will be thrown away after your interview. Therefore the only consideration is what helps you. 

Personally, I found graph paper helpful for math and for structuring neatly. 

Hi,

Graph paper helps to draw the structures better. In terms of tracking the numbers, I guess it's pretty similar.

Best

Hi Anonymous,

if you have the option, definitely go for the graph paper, will help to track zeros better (probably the #1 mistake in math) and be better organized in general with your notes.

Best,

Francesco