NULL parameter check - WHERE vs IF

Question

Currently reviewing a stored procedure to which @return_count is a parameter. Below is a standalone example of the two ways that I can set its value if a NULL is passed in.

Cant figure out if the query optimizer would treat them the same or if one method is better than the other.

DECLARE @return_count int
SET @return_count = null
IF @return_count is null
  BEGIN
    SELECT @return_count = 10
  END
GO  

DECLARE @return_count int
SET @return_count = null    
BEGIN
  SELECT @return_count = 10
  WHERE @return_count is null
END

Answer 1

You might want to consider

SELECT @return_count = ISNULL(@return_count, 10)

Answer 2

They return different exec plans, and the former is considerably less complex. The latter involves filter and scan operations. Since it doesn't appear that this needs to scale, I'd go with the former.