Seatpost mount racks are a quick and easy option for top bags or panniers (or both) and usually have a quick release so you can remove the rack when you don't want it. And yeah racks will keep you less sweaty and solves your big-pack dilemma.
I have a momentum and a medium sized chrome bag for commuting, though i dont' have any recommendations for packs for big folks. I would say to check at local outdoor shop and see what they have for you to try on. Same with the Chrome bags (which have a long main strap which should be big enough to fit around most people)
Both work fine in my experience, with pros and cons to each.
The Osprey pack (or any backpack i have used) are great, but tend to be a little bulky and hot. On the positive side: storage (esp. when full) is often preferable compared to a messenger type bag. Mostly when carrying groceries, you can more easily keep things from getting smashed.
Chrome bag is my go-to for beer runs, light commute, light grocery getting. Lots of little pockets for small items, super durable, and when properly worn sits higher and off the middle of your back, keeping you a little cooler compared to full backpacks. Also when properly secured allows you to ride pretty aggressively and keeps your pack tight against you. There's gotta be a chrome how-to thing out there somewhere, but for me the key is the little safety strap off the top main strap. this fits under your arm and when you have the main strap/buckle adjusted is very secure. (this also prevents your smartass friends from pressing your buckle and watching your pack fall to the ground. which is pretty funny except when you've got a laptop or something in there)
hope that helps.