For physics graduate school what matters more: GPA or research experience?

When applying to a PhD program, having great research experience can overcome a poor GPA (as long as it's still over 3.0, the required minimum) since a PhD is a research degree. But keep in mind not all research projects lead to publications and universities that have graduate students often prefer those students over undergrads for research assistants (undergrads really aren't useful; grad students sometimes are).