T-SQL Pattern and Exact Match Searches

I am developing a stored procedure that allows users to search by a variety of criteria. Let's say, for the sake of brevitiy, I only search one table with three fields:
Table_Application
ApplicationID  INT Identity,
DecisionID INT,    -- FK
ResponseID INT   -- FK

The stored procedure looks something like:
CREATE PROCEDURE sp_ApplicationList
   @DecisionID    INT = 0,  -- an ID of zero indicates all decisions
   @ResponseID  INT = 0   -- an ID of zero indicates all response codes
AS
  SELECT ApplicationID
    FROM Table_Application
  WHERE DecisionID = CASE @DecisionID WHEN 0 THEN DecisionID ELSE @DecisionID END
      AND ResponseID = CASE @ResponseID WHEN 0 THEN ResponseID ELSE @ResponseID END

All records in the referenced Type and Decision tables have an ID of 1 or greater. Knowing that I set it up so that if the passed in parameters equal zero all values are returned (at least for that parameter). This allows me to use a CASE statement to search for all matching records.

The problem I have is what if I have Varchar column and I allow users to do exact as well as pattern searches. In one case I have to use LIKE in the other the equal sign. Is there still a way to use a CASE statement (or any other statement to search for either exact matches or wildcards without having to resort to an IF statement?

Any help is appreciated