The argument as I've read it is that a tailpack loads the weight up high, when as a rule of thumb you want the weight down as low as possible, while on the flip side panniers create more drag than a tailpack. That being said, the panniers on my BMW were the least aerodynamic things ever designed, and I never really noticed any difference when using them. Oh, but there's a point - panniers can be a bugger for filtering. Depends how big they are, though. Personally I prefer panniers, leaves the rear seat free to tie a rolled tent or dry bag across the top. Now I just need to bodge a little frame to stop them sitting on my exhaust and melting...