I've found it more reliable to use a tape measure (the soft flexible kind, like for sewing) around the hips than the waist. Ultimately, whichever measurement is bigger, go from there, and go a size up if you're still on the verge.
Picking a larger size will mean a higher rise and more overlap in the taping... But in my experience, too small means you struggle to make the tape even reach, and that will cause uncomfortable constriction around your legs (and more frequent tape failures!)