Depends. If you wish to feed some whole grains and/or scratch grains, then buy the higher protein. If 95% of their diet will be the layer feed you provide, then 18% protein will be more than sufficient. I'd also want to read the back of the bag or the tag and see what kind of protein it is. I prefer animal fats and proteins if they are included. FWIW
I'd also compare the bang for buck. In other words, what's the price difference?