Harry is very good looking, but William (IMHO) has major sex appeal. There is something in the way William carries himself, and the fact that his body has gotten very tight in recent months, that I know if I met him in person would turned me the fuck on. Harry has a really attractive face (better than William's) but his personality put me off slightly.
If Harry and William were two guys at a bar, Harry is the hot guy that is all over you within a few minutes of meeting. After some hot and sweaty make out sessions, he takes you home and then cums too quickly, and leaves you to finish yourself off.
William is the charming guy who buys you a drink, and steadily flirts all evening. Then you get home and he rams you hard and doesn't finish until you're begging to cum, and then 15 mins later he's ready to go again. Then in the morning you find him naked in your kitchen making you breakfast.