Thanks all for your feedback. After some research and a conversation with my local bike shop, I decided to go with 35mm Schwalbe Marathon Plus tires. The rep said that the Marathon Plus tires will work fine for commuting, even in wet conditions if I exercise some common sense and caution. He also recently did the ride on the C&O Towpath on 28mm tires. He said he wouldn't recommend trying the ride on 28s, but thought that the 35mm Marathon Plus tires would handle that ride just fine.

Thanks again, and I can't wait to give these a try!