For grocery shopping, neither of those appear to have shoulder straps. You might look at the Performance grocery panniers (which may or may not be on sale, depending which catalog you look at) which do have the shoulder straps. Here's the thing, when you go into a business for whatever reason, you're going to want your stuff with you. Unless, of course, you're a very trusting person. The shoulder strap makes it easy to take stuff with you. Even if you just want to bring the empty pannier with you its easier with a shoulder strap.

Another thing about a good grocery pannier is that you can load it up with just about any backpack you want and immediately increase its capacity as well as its weather resistance. Then, in a pinch, you can wear the backpack and use the pannier for overflow.