With Ping, W is a Pitching Wedge and U is a Gap wedge.
So the W (46*) is one club weaker than a 9i and the U (50*) is one club weaker than a PW (W), or between the W and SW (which would have an S).
Here are your specs. For the W just refer to the specs for PW
PING Golf : Clubs : Irons