Maybe it's a little strong to say that the messenger bag is dead. They're still damned useful and many are, well, damned stylish too. But are we still trying to cut that dashing look by jumping on the train with an overstuffed bag or worse by carrying two bags? It doesn't say confidence.
A well crafted briefcase does. It screams confidence because its shows your transparency. You're serious. You've got work to do. And you carry your work and your tools in style.
Oh, and it says definitively that you're all grown up now. All the major houses have them. As always, opt for quality and go with leather. Let the dudes on the floor below with the three series roll with the canvas.