SQL Performance-wise, what's better: an IF...ELSE clause, or WHERE LIKE clause?

Question

I have a stored procedure that has a optional parameter, @UserID VARCHAR(50). The thing is, there are two ways to work with it:

1. Give it a default value of NULL, the have an IF...ELSE clause, that performs two different SELECT queries, one with 'WHERE UserID = @UserID' and without the where.
2. Give it a default value of '%' and then just have the where clause use 'WHERE UserID LIKE @UserID'. In the calling code, the '%' wont be used, so only exact matches will be found.

The question is: Which option is faster? Which option provides better performance as the table grows? Be aware that the UserID column is a foreign key and is not indexed.

EDIT: Something I want to add, based on some answers: The @UserID parameter is not (necessarily) the only optional parameter being passed. In some cases there are as many as 4 or 5 optional parameters.

Answer 1

What I typically do is something like

WHERE ( @UserID IS NULL OR UserID = @UserID )

And why isn't it indexed? It's generally good form to index FKs, since you often join on them...

If you're worried about query plan storage, simply do: CREATE PROCEDURE ... WITH RECOMPILE