Well the sumo, stiff legged and romanian keep the hips in the same position in general while the conventional deadlifts your hips move up and down more so.
Sumo and Stiff legged hit the hamies nicely.
Not really up on the romanian but its looks like a great overall back excercise and it too hits the hams.
Snatch grip deadlifts is another varation i haven't tried it, soon.
Its good to hit muscle groups in using different planes to strengthen weak areas, avoid overtraning and over all development. And its more fun!