for an aircooled engine i'd rather have the 2valve heads b'coz the valves seal better at high temps than a 4valve head, the bridges between the valves on a 4valve head are thin & they warp slightly when they're hot so the valves don't seat well.
for aircooled 4stroke race engines nothing beats a 2valve setup in terms of durability, in Pro Stock dragbike racing the 2valve head engines are the reigning king.