Riggle's right that there's no difference in the products (judging by just the analysis) -- all you have to do is adjust your spread rate. So, you will need more of the 18%N product to deliver as much N as the 30%N product.
But, not all is equal for the rest of the analysis. Applying 18-24-6 at 1#N/M gives you 1.33# P2O5/M and 0.33 #K2O/M, while the 1#N/M of 30-24-4 gives you 0.8# P2O5/M and 0.13# K2O/M.
So, the 18%N product might fit better for you if you need more P and K, while the 30%N product might work better if you need a little less P and K.