The first thing you should do is install a backflow prevention device. The only thing you would gain be going to the larger line is a little less friction loss over a long run. 3/4" is very marginal for supplying a water system. But you must protect against cross contamination of your water supply...both yours and the city's. I would advise you to get a professional sprinkler company to go over you plans and recommend the best course of action. You could waste a lot of time, work, and money to put in a system that doesn't work the way it should and risk the health of your family and neighbors.