Great advice as usual here. That's why I love coming here. I learn so much.

Surg - I commuted last year with a Camelback Mule. (Thankfully, it was on sale. ) Loaded with fluid, it worked out great. I could carry undies (I wear OR scrubs too so no need to pack clothes), phone, snacks,... but not enough room for a lunch too. Working at the main hospital, I just ate at the cafeteria instead of carrying food. They even have fresh sushi everyday there. I could eat for under $6/day. This year I mostly work at our clinic at the waterfront. There is 1 upscale-type of cafe in the building (way too expensive) so we all pack our lunches. Kit and I bought grocery panniers at Performance and they are a nice size but have no protection at all from the weather since they are totally open at the top. We like them though because we can sling them over our shoulder and put the groceries right in without needing other bags. Then just clip straight onto the bike and go. We use them on weekend trips (driving) to the coast and such. Very versatile but not weather proof at all.
I've been looking for some sales on a rainproof one (like Ortleib) about that size or bigger for my commute. We also want them for some touring later this year. With the addition of motorcycles in our family, we are also thinking ahead on using them that way. We see motorcycles and scooters with them more and more these days. Orlieb and Arkel, while being pricey up front, seem the most versatile and therefore the best buys. You can pop a backpack harness on too and use them everywhere you need to be.
As for riding with just one, the only time I notice it is when I've loaded the bag and then take my bike out of the rack. That was a circus. Almost dropped my bike the first time. I've learned to unlock my bike first, then load it so I can keep my hands on it until I back it out.
Many people have a problem with their foot hitting the bag when they pedal. Get a rack that is adjustable so you can slide them back if needed. Some are angled back to help with the clearance too.

Hope this helps.