Alloy pulleys are for the layshaft, RW's are for diffs.
I use an alloy pulley because of durability. Plastic pulleys wear faster, worn pulley can cause belt skipping. There was also an issue with the early plastic pulleys breaking around the holding pin, making your car a no-wheel-drive one, but this has apparently been fixed by redesigning the pulley.
RW pulleys are nice, they are very durable and a bit smaller than the originals, making belts run more freely. The standard pulleys (both diff & layshaft) are OK though, I know guys who use standard items and are faster than me (which is not too hard to do)