Usually the size of the area you are covering is what determines whether to use sprays or rotors.
And the reason to use 1" pipe even if the supply line is smaller than that is to lessen the amount of friction loss. You want to have as much available water to work with as you can. A 1" pipe will lose less than a 3/4" over the same distance.

Thanks Dave,
Yes, I figured that the area to be covered would have much to do with it, but I have been successfully using small Hunter rotors in relatively small areas that could be supplied by sprays. I assume that the rotors have to stay on (watering) much longer and use less water/time unit because they are only watering a narrow band at any given moment. True??? But, say you are trying to water a 12' diameter, which would provide the more even watering from the riser to the periphery of the 12' circle? And be the more maintenance free?