Here's the pros and cons of the two:
Pros of the S45:
It's $50 cheaper.
It's smaller.
It's a newer camera, which uses the same internal electronics as the G3. Therefore, is has more advanced focusing capabilities, a better movie mode, a more powerful optical zoom, and a faster image processing time.
It's easier to use.
Cons of the S45
A weak flash, and no ability to attach flashes and lenses.
Pros of the G2:
It has a flip out and swivel LCD screen, like the newer G3 model.
It supports flash and lens attachments
The larger case is easier to hold onto.
Cons of the G2:
Slower image processing time.
If you pick up a model with older firmware, you might need to update it to get full Windows XP and larger CompactFlash card support.
Highly limited movie mode.
The megapixel and optical zoom ratings are the same on both cameras. Both of them both of them are good, but I picked the S45 due to the smaller size.