Which GPA (cumulative or major) is more important for graduate admissions in the US?
First of all, remember that GPA is more of a negative filter than a positive filter.
If you do not have an excellent GPA in your relevant major, then you are definitely going to have trouble getting admitted.  The GPA outside of your relevant major is less important, but it will still raise major questions if you have, say, all As in major and all Fs outside.