I've never pulled sumo before, and i tried it in training for the first time, and matched my best PR for conventional, on my first sumo ever. I've never really liked the idea of sumo, I always thought it was kind of cheating "real men pull conventional" but this is stupid thinking, from a competition stand point.
For me on the next training cycle, I'm going to lower the volume a bit, and do both styles of pull at every weight -warm up-max weight-back down, so if all that added up to 10 sets, I guess I'll be doing 20 sets. This should give me an idea of where I'm at, and which style I'm stronger in. 531 is only 3 week cycle so I'll know at the end of 3 weeks what lift I want to push forward on. This is what I'm doing personaly, I don't know if it will help in your choice, but goodluck.
First time I pulled sumo I pulled 20 lbs over my conventional when I was pulling in the low 500's conventional, just felt natural to me. Did a great deal of deficit conventional pulls all the time before that. Makes sumo seems like a rack pull.
This is pretty much exactly my experience. I think many people who are built for sumo don't even realize it. I personally train both in three-week alternating sub-cycles and feel as though they both play off of each other.