What is the execution plan?
I think both are equal for SQL Server so no matter you will use the execution plan should be the same.
One thing:
If you store time potion in that datetime field and you want date between 01/01/2008 and 01/31/2008 you have to use:

SELECT ...
       FROM ...
WHERE DateTimeField >= '20080101' AND --- 01 Jan 2008
      DateTimeField <  '20080201'     --- 02 Feb 2008

Just because 01/31/2008 01:03:00.111 is bigger than just 01/31/2008.