I was gonna say thigh because it's more easily accessible than lower back, but then you'd probably have to take off your pants or w/e you're rocking below the waist to get at it, which would be awkward in public. Lower back would be slightly better in that regard and might be able to provide some structural help to the spine, so I'm gonna say lower back.
I'm gonna say thigh. My body is so janky my first thought is the lower back storage would keep falling open and spilling my stuff out behind me. I could keep the thigh compartment shut with thigh high socks and wear a loose skirt so I can get to it fairly easily. And I could use it as a cup holder on long drives.