P510 vs s9300 - 40x vs 18x ?
If you didn't have the s3100 then I'd say the S9300, but since you have a pocket take everywhere camera then you'll probably regret not grabbing the bulkier but rather excellent P510.
Having said that I went up the street today with my S9100 because it's convenient and a lovely camera to use, I have to think the S9300 is a bit better again but the > P510 gets stuff you can't even see.
But if he can't see it he won't miss it will he
'I don't necessarily believe everything I say'!