I'm 6'1. When I started with conventional, even though my back never rounds because of my flexible hamstrings, I would use my lower back way too much (very little usage of the legs).
I then switched to sumo, and although I was a bit weaker in the beginning (and especially at the bottom of the lift), after 4 weeks of power cleans and dumbbell snatches in my program I'm pulling WAY more with sumo and with full speed. I power clean with conventional stance.
I'm able to utilize my legs, glutes, and posterior chain in general way more with sumo, but with conventional I use my lower back way too much.
My opinion is that if your lower back is getting so pumped to the point that you have to sit down and stretch it out, you're using your lower back too much. You'll also limit your poundages in the long run if you don't use your legs.