Why Minor Diff Makes Big Lag

The DB Enginee is SQL2005 SP1. In MSSMS

the following query always finishes within 1 minute

SELECT *
FROM t1 INNER JOIN t2 ON t1.key= t2.key
where t1.StartTime >= '3/19/2008' AND t1.EndTime <= '3/20/2008'
AND t2.StartTime >= t1.StartTime AND t2.StartTime < t1.EndTime

however, the following always takes 10+ minutes
declare @pStartDate datetime
declare @pEndDate datetime
set @pStartDate='3/19/2008'
set @pEndDate='3/20/2008'
SELECT *
FROM t1 INNER JOIN t2 ON t1.key= t2.key
where t1.StartTime >= @pStartDate AND t1.EndTime <= @pEndDate
AND t2.StartTime >= t1.StartTime AND t2.StartTime < t1.EndTime


Both return same result. The only diff. is the blue part. Why the second one performances so badly?
Thanks.