Hi,
I recommend to use the r3 (1280 x 720 50 / 60 Hz) setting.
This setting creates footage in the now practically standard 16:9 format. The high frame rate of 50 / 60 Hz reduces the annoying wobbling effect, which you can see on many videos and which is caused by vibrations.
As the whole optics of the GoPro isn't too sophisticated, this setting doesn't cause any
visible reduction of the video's resolution compared to the 1920x1080 25 / 30 Hz setting.
If possible, I would not use the suction mount outside of the aircraft. While it is generally able to handle the airspeed of a typical GA plane, you can never be 100% sure that it doesn't leak after a while because of some minor dirt parts under the sealing surfaces.
If you lose it - bad luck. If you lose it and it hurts somebody, you're f----d - and the video evidence is laying right next to the casualty.
The suction mount also tends to vibrate in the airstream, what again causes wobbling effects.
I use therefore a home made, strong clamp for the wing strut and / or one of the adhesive mounts, after cleaning the surface with window cleaner. The adhesive mounts can be removed be cutting the foam layer with a thin cutter knife, without leaving any trace.
Here are pictures of my installations (scroll down, right side):
http://vimeo.com/18855219
Cheers,
Oliver