"Bro...What is the better choice if I want to develop my back using the deadlift - sumo or conventional?"
Conventional. There is no doubt about it. The sumo deadlift is much more leg dominant. You are also very upright which places more stress on the lower body than your back. When you perform a conventional deadlift there is more stress on the back which in this case is a good thing since we are looking do develop it. At the same time the pull is longer which means that the muscles are stressed over a larger range of motion. The wide stance which comes with the sumo deadlift reduces the “effective” length of your upper leg and you end up doing the exercise over a smaller ROM (range of motion). As the great Mike Mentzer has said: “Exercise is about movement. The greater the movement, the greater the exercise.” So if your goal is to develop your erectors, traps, lats the conventional deadlift will do a better job than the sumo deadlift.
Jouko Ahola /strongman/
You are looking at the back of 400kg deadlifting machine.
"Ok..but why are so many powerlifter using the sumo deadlift?"
Reason number one why you see so many people doing sumo deadlifts is - you can lift more weight because the movement is shorter and there is less back strain. Since the goal of every powerlifter is to lift more weight some of them go for the sumo style. Some don't. Also your body type is largely important what kind of stance you will prefer.
"Hm...what kind of bodytypes are good for conventional stance and respectably for sumo?"
It depends. I think most people will have to try both exercises and choose one. I prefer conventional deadlifts because when I experience sharp hip pain when I lock a sumo deadlift. It's due to poor flexibility. I think that if you have the “deadlift machine” body type - looong arms and somewhat short legs (kinda like an orangoutan) you should stick to the conventional. But anyway if you have that bodytype you will be a deadlifting machine no matter what you do.
On the other hand there are people with normal to short arms and somewhat long legs. You mind find yourself in trouble because your starting position may cause you to raise your hips so high that your back is almost parallel to the floor at the beginning of the lift. In that situation the sumo deadlift might be an option since it reduces the “effective” femur length and you are able to start in a more upright position. People who share those characteristics find themselves more comfortable with sumo BUT that's not always the case. I have a similar bodytype and I still prefer the conventional deadlift even though I may end up lifting less weight. In the end of the day you should try both and stick to what feels better. Your body will be your best guide. Not me.
"Ok...but what deadlift should I choose when I want to develop my legs?"
Technically the sumo deadlift hits your legs much more completely than the conventional. There is more quad and adductor involvement. Also your glutes work from a different pulling angle obviously since you will have to use a wide stance. However unless you use the Romanian deadlift to hit your hamstrings I think there are much better leg exercises than both movements because the range of motion is quite short compared to a squat or even a proper leg press. Your muscles will report faster growth when you train them over their full range of motion. Do deadlifts for your hamstrings but keep in mind that the squat and its variations might be a better choice for your quadriceps workout.
"Can you give me examples of successful sumo deadlifters?"
Ed Coan, the most successful powerlifter to this day, is the first that comes to mind. He set a lot of records using that very same style but later on switched to conventional due to an injury.
"You said that the conventional deadlift offers the longest range of motion. Is there a way to increase it even further?"
Yes, there is. Try deadlifting using small plates. Instead of using the regular 45 pounds go for 35 lbs or even 20lbs and enjoy the pain. You won't be able to deadlift as much obviously but it will feel way harder.
Another way to torture yourself would be to perform snatch grip deadlifts with small plates. No need to ask if it hurts.