Richard Harding Davis, my favorite war correspondent (circa 1900) carried a full size dbl. action revolver. After the many engagements he was 'close to' he thought that 6 was sufficient and a reload unlikely. As right as he may have been, it is little comfort when number 7 would have saved your bacon.
After thinking about the motor skills involved under tremendous (ie, fumbling) pressure, I think the back-up gun is the best reload. Something small and unemcumbering, that you never notice is there, but with some serious loads. In the last ditch its going to be up close and personal; a snubby should be fine.