Speaking of heel strike - when I wear my pumps - especially the 4-inch heels - I find pedaling to be tricky. Two things about hitting your panniers while pedalling - make sure the pannier that you get doesn't have little front-projecting pockets - and set the bags as far back as possible on the rack. To prevent slow forward crawl - which happens while braking with all but the best attaching systems - use either duck tape or a broccoli rubber band in front of the hooks - not pretty but cheap and it works.

If you are only going to use one - I'd consider a rack-top bag instead.