I don't think you'd really want either. Defenders need to be able to outrange other hounds, since you don't have much speed. You're better off with rifles or cannons.
If you had to pick, I would select shotguns, but not use them much. Shotguns may not do the continuous damage of a machinegun, but its a lot easier to tag a scout with a good shotgun blast than with a machinegun. For anything else, I would be using a heavier weapon, such as a cannon.