My thought is if you open the phone case, you could stitch along the purse contrasting band seam line and across the top of the phone case just at the top of the opening. This way you could attach it in either position. For the first position, attach it to the top seam line just below the handle; for the second position, attach it at the bottom seam line at the center. This way, you wouldn't have the seam showing inside, as it would just be more stitching over an existing seam.
For me, I would want the centered position due to the balance issue.