I think you should try both and be able to do both to some extent. But the question is whether you want to pick the form that simply allows you to lift the most or pick the one that gives you the most bang for you buck and, frankly, gives you more hypertrophy.
If you are a powerlifter who can pull more at sumo's then you answer is simple. Sumos decrease the range of motion and if your body type allows it it simply requires less work. Less work isn't necessarily good if your looking to be strong and big in a overall way. For most people who can do them and are not competitors I thing using conventional for their main form of pull is going to give them the most bang for their buck. Sumos use similar muscles to wide stance squat...almost exactly the same.
Given all that, no matter what style you choose for your bread and butter in the long run, I disagree that someone who is inexperience with deads and who hasn't been doing a lot of wide stance anything should start out with sumos no matter how flexible you are. I don't see why he should recommend sumos in general just because he does them as this tends to be a bodytype and preferential thing rather than a one is better thing. But I have a feeling, and my feeling are usually right in the long run
, that going staight to sumo is going to beat the hell out of your hips if you are inexperienced because the forces on the hip joint are much greater.