Select statement vs StoredProcedure

Hi, It is a SQL 2008 R2 server. I have a very large table with more than 2 million rows.

I have a procedure:

BEGIN TRAN

CREATE PROCEDURE Test
@StartDay DATE ,
@EndDay DATE ,
@xCreateDate DATETIME ,
@xUpdateDate DATETIME
AS 
BEGIN  
 -- SET NOCOUNT ON added to prevent extra result sets from  
 -- interfering with SELECT statements.  
    SET NOCOUNT ON ;  

-- Insert statements for procedure here  
--declare @s smalldatetime;set @s = cast('2011-03-01' as smalldatetime);  
--declare @f smalldatetime;set @f = cast('2011-03-31' as smalldatetime);  

    SELECT  *
    FROM    Testtable
    WHERE   ( InvoiceDate >= @StartDay
              AND InvoiceDate < @EndDay
              AND ( ( @xCreateDate = '' )
                    OR ( xCreateDate > @xCreateDate )
                  )
              AND ( ( @xUpdateDate = '' )
                    OR ( xUpdateDate > @xUpdateDate )
                  )
            )  
END  

--SQL statement:

SELECT  *
FROM    Testtable
WHERE   ( InvoiceDate >= '7/1/2008'
          AND InvoiceDate < '8/1/2008'
          AND ( ( '' = '' )
                OR ( xCreateDate > '' )
              )
          AND ( ( '' = '' )
                OR ( xUpdateDate > '' )
              )
        )


ROLLBACK

Below are the execution details when the SP is run as opposed to the select query being run:

When it is run for July data,

sql statement - 2273991 rows - 9 min 18 secs - Execution plan:table scan

stored proc - 2273991 rows - 2 min 35 sec - Execution plan:RID lookup, index seek (invoice date)

When it is run for Aug data,

sql statement - 1920308 - 2.26 - RID lookup, index seek (invoice date)

stored proc - 1920308 - 2.23 - RID lookup, index seek (invoice date)

Questions Why is there such differences in the execution times and execution plans?

There is an non-clustered index of invoicedate which is used when running the statements for Aug data.

Can someone help me on this??

Thank you in advance..