I think this is a close fight either one. Both are/were extremely strong lightweights, Castillo for my money might be a tad more durable, but Mancini was MUCH quicker, and more active. Castillo is more accurate however, with perhaps a tad more defensive skill(which aint saying much). I could see Mancini outworking Castillo, however I could also see Castillo busting Ray up. Pick em IMO.
Day-before weigh-ins, and Castillo fucking murders him. It's not even an issue. Castillo would have been a welterweight or jr. middleweight in Mancini's era.