I would look to adding a good flash to your Canon, but decide if you need all the bells and whistles of the 550, if not go with a 3rd party flash for half the money and think about picking up some used MF gear--prices are insanely low. I've gotten alot of mileage out of my TLRs over the last few years and am now actually dumping some of my 35mm gear to fund my new/used Mamiya 645 system. These are hard choices, but the difference in image quality is well worth it.

Sean