Some coaches believe that the stance is something genetic, like the color of your eyes. Lifters come as sumo deadlifters or traditional. It’s a joke, of course, but not that far from the truth: you will notice that elite lifters very rarely shift from one to the other.
There is a general rule that short lifters with short arms (me, for example) will benefit from the sumo stance. Therefore, will “naturally” choose it, while tall lifters with long arms will do the opposite. While this is statistically true, the exceptions are more important than the rule: why do a few extremely accomplished tall deadlifters choose the sumo stance and some incredible short ones choose traditional?
The same question has been asked about differences on the squat. Take a look at this excellent article on the subject.
The Best Kept Secret: Why People HAVE to Squat Differently
The truth lies in your internal anatomy.
So how do you find out what is “your” stance?
My advice is always to try a lot. One way of doing that is training the same weight alternating the traditional and the sumo stance.
You can do it alternating sets or, in the same set, alternating reps. Say you choose to do sets of 4: do 2 reps traditional, 2 sumo, alternating their order (otherwise you can think one is harder than the other because you are more fatigued when executing that one).
At a certain point, you will probably feel one works better than the other, sometimes in a precisely quantitative way: you can lift more weight on your preferred stance.