Trying to decide between the two cameras, any opinions greatly appreciated.

After much research, I chose the SD550 for its excellent image quality, ease of use (very intuitive), decent battery life, 2.5 LCD and quality, quality build and design. You cannot go wrong with Canon and this is one of their most popular P & S cameras - with good reason. I handled both cameras and thought the Nikon 7900 was too small for my tastes. Comparing the two side by side and reading review after review convinced me to go for the SD550. Hope this helps.