Even if you were to pull Sumo in competition I probably wouldn't do the majority of your training with Sumo, I'd keep SLDL (done heavy) as your main assistance. Certainly if your form needs work and you KNOW what you're trying to work on then fine, drill the Sumo. That's a key point as well, people talk about drilling volume on a lift to correct form BUT if they don't actually know what in the hell they're trying to correct then it's futile. But I see that all too often.
For the majority of your pulling I'd smash the crap out of the SLDL.