just go to the canon website.
both are semi pro

it depends what u want.. generally, most people prefer the s3is
G7's image quality is superb. it has 6x optical zoom
its built quality is superior. but personally, i think its too expensive(for a non dslr camera)

s3 has superior zoom (12 x) a excellent movie mode, packed with features and i think its a better bargain..
just my thoughts.

both of them are prosumer cameras
s3is not a normal p+s camera
g7 does have DIGIC III while s3is uses DIGIC II
but note that s5is will use the DIGIC III imaging processor

ok
A series.... can have 3-6 times optical zoom
takes AA batteries. good P+S camera
special modes available for some models
great bargain for most models. its bigger than compact cameras but still small enough to fit in pocket. a in between type of camera
A640 10 Mp plus articulated lcd is a great camera. A710 is also a great camera with IS.

G series. Those who want manual control without buying a DSLR camera.
produce great images. 6 times optical zoom and 10 MP. great camera with hefty price tag too. not as good as a dslr but still king among the non dslr.
camera ; G7

S series
ultrazoom camera. 12 x optical zoom. have manual control and made easy due to dedicated buttons. S3is has 6 mp while the up coming s5is will have 8mp plus hotshoe. Articulated LCD so u can swivel it. comes with IS and a quiet zoom. Great movie mode too but not advisable if u want poster size images.
camera :S3is

just my thoughts... i dont mind suggestions and criticisms.