A650 looks like a nice choice... if you put the RAW hack it gets than better closer to the g9... to shoot pictures from strange positions above the surface the display is very neat.
But what made me decide for the G9 was the dial control. I couldn't have anymore a manual focus that works by pushing buttons "tap, tap, tap, tap"
Of course, if you want to buy the G9 with the canon housing, better choose the A650; because someone in canon forgot to make a control on the housing for the dial control.
I know about the "shortcut" to operate the same funcitons from the dial control, but it is again "tap, tap, tap, tap" with an extra annoyance; you have to keep pressed the shortcut button.
What I don't know, it is if the A650 has the same nice post-processing options of the G9.
If you can get hold of the manuals to compare you can best see what camera has all you need (or more)
Comparing with the tool from canon. it shows this differences:
- G9 can use external strobes (if you use another housing and not the canon one). Which helps saving battery, not to speak of better colour in your pics and less money in your pocket.
- G9 lacks that flexible display which I don't miss underwater and above surface I am used to live without it (I don't even make much pictures while being dry).
- G9 forces you to use canon batteries. I hope it is enough for two consecutive dives
- A650 lacks "Continuous focusing"; nice feature if I may say so; it is battery consuming but helps keeping focus of a moving target.
- A650 lacks focus bracketing; I haven't yet manage to find that usefull with subjects close to the camera (I love macro).
- A650 can put the shutter speed down to 1/2000, the G9 gets 1/2500 (not sure if I ever will be able to enjoy that :-?
- G9 has dioptry correction for the viewfinder (again, another surface feature)
- G9 has more modes for while balancing
- G9 display is just bigger, has more dots and brightness can be adjusted (it helps to don't miss the moving display of the a650).
- G9 internal flash has more range than the A650, but in macro it makes a not so nice shadow of the objective over the subject.
- G9 photo effects, has more and the custom mode is not limited. Nice to play with, practical... who knows?
- G9 can be faster in continuos shooting (if the LiveView is off)
- A650 is 20g lighter :-D
- Last and most important: G9 is more cool 