First off when adding a 48" to your 36" you would be only saving a little time.
Also Most 4' gates are exactly 4' and a 48" mower will not get through those gates so you will be having to still use the 36" for those back yards. So the 48" will not increase your production enough to make the purchase that worth while.
If you had 30 acres to mow every week.
36" would take 22.9 hours to do. This leaves out drive time, and all other times. Just time to mow.
48" 17.1 hours
54" 15.3 hours
I would go with the 54" because it will give you the biggest increase in productivity. And many new gates are 5' and a 54" will fit through a 5' gate. You will still have to use the 36" for the small gates. The same gates that a 48" would not fit through as well.