You better go with the Leghorn, if egg production verses cost is important to you.
Even if the SF laid 4-5 eggs per week... they are more medium sized eggs and much heavier birds so they WILL eat more and make less eggs. Leghorns lay fairly large eggs and VERY prolifically. SF also grow and mature A LOT slower. To me they are beautiful hobby birds but will NEVER come close to leghorn in laying.
And quite honestly, despite our leghorn not being held and loved on per her own wishes... she is one of the coolest birds to see with her giant red comb on her petite white body. She is very entertaining and a great part of the flock (when we had her). Everyone who came over wanted to see her regardless of the "rare" breeds I had on pasture! Leghorn aren't nearly as plain as they seem!