Just got a knog hot dog pannier/messenger bag and I love it. The strap is super wide and soft, and while the universal hub allows it to be used as a pannier I wouldn't trust it with too much weight. Still it's really convenient overall to have it do double duty and look good.

If you don't mind a little inconvenience you could use a Zimbale saddle bag and take it off and on the bike. The leather and canvas looks classy and you can buy a strap that lets it be used as a messenger bag.