If that's the case, then it would seem that the improved camera will be a big improvement for you and that the improved processor is probably less of a draw for you. However, it was identified in iOS 5 beta 1 that the image editing features that are new to iOS 5 will not be available on the older 3GS handset, so that is an additional incentive for you to upgrade as well.
Since games are not a big part of your usage, you probably won't notice the better graphics processing, though the overall quickness should be improved a bit. However, you certainly should appreciate the higher screen resolution that was introduced with the iPhone 4 and continues with the 4S.
In short, you could get by for another year without the iPhone 4S. If you feel like changing your carrier within the next 12 months, then upgrading would be a bad idea. Some people really want LTE and so they might want to wait for the next handset next year on a different carrier.
But I think it's very safe to say that there are enough upgrades and enhancements over the last two years to justify upgrading from a 3GS to 4S.