This is really an interesting question! And I speak from experience given that I am one of the few on this board that currently uses both! While I've only had my S3 a couple of weeks it is V-E-R-Y easy to use.
Pros of S3 (vs. G5):
1) Easier to hold in my hand than G5
2) Like having ISO button right on camera body
3) If pre-set options are to your liking, there are tons! I like the super-macro! G5 has very few pre-sets.
Cons of S3 I've found so far:
1) AA batteries vs. lithium of G5. My lithiums lasted weeks of use!
2) Auto setting stinks 90% of the time so far for me! vs. Auto on G5 which produces fantastic pictures 80% of the time!
3) I miss the internal flash; lose time popping up the flash manually.
4) The Movie button is awkardly placed. I hit it accidentally several times in the first day.
G5 in answer to your questions above:
I have not been happy with most of my fireworks shots but I don't work at getting them either (LOL). I do have a few that I like. I have also taken some sunset shots on Auto and used PSE to gently lighten as needed. See below! I love the Zoom on my G5. The bonfire pictures were taken from at least 50 yards away!
I don't have comparison shots to show you and I can tell you that I chose the S3 for it's easy settings etc. and newer features that my G5 doesn't have! Cost was a minor issue since my S3 was basically free using
Amazon certificates.
I will keep using my G5 for a while at least! I love snapping pics and later zooming in on something when I'm processing the picture in PSE!
And ukcatfan! - The G5 does have a Movie mode. I have some clips of my niece.
If you think of any not too technical questions, please feel free to ask!