I would say do both. You cant put oo much of this stuff down.
Once the lawn is established with organics it will be pretty much self sufficient. Making almost all its own N and need little P and K.
Dont forget to mow high too this really helps.
Did you do a soil test yet?
I manage our retail division as well. Let me know if you cant find it and I will get you the 10lbs that you need to put this stuff down.