Conventional vs. Sumo Deadlifts

First and foremost, conventional deadlifts and sumo deadlifts are NOT the same lift, and they do not/should not feel the same. Secondly, conventional is interchangeable with traditional, and sumo may sometimes be referred to as wide stance. 

My "home" is conventional. I absolutely thrive doing conventional deadlifts, and I feel most comfortable in that set up. I can still do sumo, but it just doesn't feel as right as conventional. This is pretty common for many lifters; not necessarily that their comforts are found in traditional deadlifts but that most lifters tend to really thrive in one or the other. There are many reasons why one variation might feel better than the other; flexibility, skeletal structure and distribution, height, weight, different strengths and weaknesses among activated muscle groups, etc. 

The main differences between the two are in the positioning of the feet and hips, where your center of gravity is relative to your feet, and in the actual pull with regards to which muscles are emphasized and when. Let's break both down into their respective set ups and pulls:

THE SETUP - CONVENTIONAL

FEET: vertical jump distance apart, which is usually pretty similar to hip distance, with a slight turn out (to a degree that is comfortable for you)
HIPS, KNEES, BACK: hips will be behind you with the knees bent and the shins touching the bar, and the back will be at whatever angle is necessary for this position to occur and it will be in both thoracic and lumbar extension
SPINE: neutral, and rigid, and this includes the neck (most people have a tendency to break at the neck and look upwards)
CHEST: puff that baby out and be as open as possible (this will help extend the thoracic spine)
ARMS, SHOULDERS: keep the shoulders pressed down and back to maintain an open chest (this will also activate the lats), place the hands just outside of the legs, and just before the pull align yourself over the bar so that your scapulae (shoulder BLADES) are just above the bar -- this will put your SHOULDERS slightly IN FRONT of the bar
THE BAR: keep it so tight to your shins it's touching them (this provides a mechanical advantage when you pull, because the closer the weight is to your center of gravity -- which is approximately at your mid-foot--  the easier it will be to lift... imagine holding 20 pounds out at arms length, and then imagine holding 20 pounds down by your side, which is easier?)
CENTER OF GRAVITY: should be above about the middle of the foot, and the bar and your scapulae should both be above mid-foot prior to pulling

THE PULL - CONVENTIONAL

PULLING OUT THE SLACK: set the neck in a neutral position, push the shoulders down and back towards the pelvis to open up the chest and activate the lats, and pull up slightly on the bar to eliminate any slack -- everything should be tense and turned on in this moment right before the pull
PULLING: you want your hips and knees to extend simultaneously, and ideally, you want this extension to begin as the weight breaks the floor (vs the hips rising and back angle changing BEFORE the weight leaves the ground), and as you pull you want to keep the bar tight to your body
AT THE TOP: shoulders should remain unshrugged and pulled down and back, chest should be open, and arms should be at a dead hang -- watch out not to overextend the lumbar spine at the top of the lift (squeezing the glutes will help with this, and really should always be a part of the deadlift)

Okay, so how does this compare to sumo?

THE SETUP - SUMO

FEET: well outside of hip distance and externally rotated -- the "snatch" lines on an olympic barbell are a good landmark to start with, and you can adjust based on comfort from there
LEGS, KNEES, HIPS: legs and knees should be turned out towards the weights on the barbell, but rather than shooting the hips backwards as in a traditional deadlift, you want to dip your hips down to the bar while maintaining maximal external rotation of the femur
SPINE: neutral, and this includes the neck (most people have a tendency to break at the neck and look upwards)
CHEST: puff that baby out and be as open as possible (this will help flatten out the thoracic spine)
ARMS, SHOULDERS: keep the shoulders pressed down and back to maintain an open chest (this will activate the lats), and grab the bar with hands at shoulder distance apart
THE BAR: keep it so tight to your shins it is touching them (this provides a mechanical advantage when you pull, because the close the weight is to your center of gravity, the easier it will be to lift -- imagine hold 20 pounds out at arms length with a straight arm, and then imagine holding 20 pounds down by your side, which is easier?)
CENTER OF GRAVITY: should be back towards the heel of the foot as the torso will be more upright 

THE PULL - SUMO

PULLING OUT THE SLACK: set the neck in a neutral position, push the shoulders down and back towards the pelvis to open up the chest and activate the lats, and pull up slightly on the bar to eliminate any slack -- everything should be tense and turned on in this moment right before the pull
PULLING: push your knees away from you to maintain external rotation of the femur as you stand up and pull your hips through to the front
AT THE TOP: shoulders should remain unshrugged and pulled down and back, chest should be open, and arms should be at a dead hang -- watch out not to overextend at the lumbar spine at the top of the lift (squeezing the glutes will help with this, and really should always be a part of the deadlift)

Here is a video breakdown of the differences described, and you will be able to see the actual pull for both traditional and sumo deadlifts in the video. (Thanks, Elliot!)

FINAL REMARKS

GRIP: at some point, grip will become a limiting factor in the deadlift -- an over/under orientation of the hands can be adopted at this point in which one will grab overhand and the other will grab underhand (if you're unsure of which hand to switch to the underhand position, try standing above the bar, and lowering to it quickly without thinking and allowing which ever hand naturally switches to switch... it's usually the non-dominant hand)

VARIATION WITH GRIP: over/under grip should really only be employed once grip becomes a limiting factor, and, in addition to that, should be varied often -- if you ALWAYS pull over/under with your left hand under the bar and your right hand over the bar then you are apt to develop some imbalances in accordance with that set up, so even if you just switch for one rep out of every 10 to 15, it's definitely a worthwhile effort to make in the long run

VARIATION IN GENERAL: it's always ideal to vary the lifts you do in the gym, and especially if you're a serious lifter, you'll likely benefit more from practicing BOTH deadlifts rather than just picking one and only practicing that one

Happy deadlifting!