Susan I'm sure you will love sewing on your 9450...was thinking of you yesterday when sewing on mine!  Keeping your 6600 as a backup is a good idea if you are sewing with different coloured thread and/or stitch etc.  My backup machine is a Brother PQ1500S (now with LED lighting, so technically an SL) and when I'm making jeans I do the seams on the 9450, seam finishes on the overlocker and topstitching on the Brother, this makes the whole process much quicker as you aren't re-threading etc for each stage, same goes for bags/jackets/etc.

Have fun ;-)