They usually have the labels on the bottom of the bag that you can compare side by side. Unless there is a major difference in the labels I go with Dumor, I have found my Bobs do like the grains in the Purina but at 50% price premium I've been mixing 33% Purina with 67% Dumor.
If I recall correctly the Dumor chick starter is 24% protein, and the bag says feed um this 6 weeks, then the starter grower til mostly grown. The Dumor Starter/grower is 20% protein, and the Nutrena doesn't have the starter, just the starter grower at 18% protein.
So, it kinda depends on which products your looking at, Purina recommends 30% protein for the first 6 weeks, then a 19 or 20% protein depending on layers or flight conditioner. Based upon this I'd say that the Dumor is best for the first 6 weeks (after the Purina starter), then it becomes a bit more confusing but the Dumor starter/grower is my choice (but i've been looking at the purina personally too).
Do you remember if it was the starter or starter grower you used before?