I had a G11 in a Patima aluminium housing and just bought a G12 to use in the same housing.
The decision for the G11/12 was the bigger size as i get cramps in my hands using the S90/S90/S100 on land.
The most important difference between the S and G series is that the G series has a hotshoe for a strobe, providing
full TTL or Nikonos strobe triggering with a housing equipped with a electrical bulkhead.
If this matters for you is your decision, some like the less complicated optical triggering,
some (especially persons who are used to shoot manually) like electrical triggering.
Another "important" difference is that the S100 has Full HD 1920x1080 Video and can zoom while recording,
but the videos made under water with such a small camera usually are very shaky
Yes the wider lens is a improvement on land but if the lens extends longer it reduces the wider angle due to vignetting.
Patima solved this problem on my housing as i got 2 ports, one short (but still flat) WA-port and a longer zoom port, but it's vignetting
on WA.
Chris