when i did my underside ben i used hammerite underseal with added waxoyl.
you can buy it in halfords.
it comes in spray and brush form and i used both.
baremetal the underside,then prime,then hammerite.
i used stonechip in my arches.
if you stonechip the underside you risk future rust probs i think.
stonechip dries hard,and if i gets chipped,you risk water getting underneath it and rusting the car.
hammerite underseal stays like rubbery,and if it chips it kinda moulds back into itself and seals again.
depends what you want really.
stonechip would prob look better and be easier to clean ,but underseal makes car more useable.
cheers
steve