Its a tough question. Maybe both. When on the keyboard dock, for typing, it makes sense to have it in portrait mode. Its also more convenient if you are plugging in the sync cable while leaving the iPad in a portfolio case to have the plug on the short side, bottom (bottom being near the close button).
However, the iPad makes a great photo frame and video player. Both of those functions make sense to have a long side plug. Maybe if the dock would allow you to fold the connector down and use it as a stand and include an optional short cable with a 90deg connector you could run up to the plug while its in landscape.
Although, with the excellent battery life, you dont really have to worry as much about it always being plugged in. Mine only gets plugged in while I'm sleeping.