My friend and I went through this already... Other than size, the G9 came out ahead.
this was input in another thread:
My buddy asked me to accompany him to the camera store last week to purchase a new camera. I pointed him to the G9 and asked for a Nikon equivalent. The store owner brought us the P5100. In comparison of the P5100 to the G9, the G9 won in almost every aspect except two:
1) The P5100 was more comfortable to hold (integrated grip) and smaller in size
2) The price
My friend decided on the G9 for his more "serious" photos and will still keep his older kodak for a more pocketable snapshooter. My personal feelings is the G9 is a DSLR like camera in a compact package... complete with RAW and almost every feature that entry level DSLRs have built-in. It also has a longer zoom than the nikon. I really wish Canon would design a compact swivel E-TTL flash with the Gseries in mind so that the G9 + dedicated flash is still easy to pack (420EX is bigger than the camera). By comparison, the P5100 is just another P&S albeit a good one.