Based on past experience, getting full-hookups is not first come first served. It is based purely on chance, which section FMCA is filling when you arrive.
Based on this thread, it seems that the advantage to volunteering is that you will have to stay longer, so the cost per day goes down, not the total cost.
The advantage to booking earlier is that you have the rights to pay $380 for a 50 amp site before they run out.
Yes there will be honey wagons and water fill services for a charge.
Have fun!