I don't know about the i850 but I've got the s820 for a little over a year now. Overall I'm quite happy with the quality of the photo prints. You can see grains if you REALLY look at it up close, but then again who on earth looks at a photo 5cm away from their eye.
Come to think of it, if I waited now I could have bought the i950 for the same price that I paid for my s820. I've heard / read many people praising how wonderful the i950 is.
What sort of prints are you doing? From what source? I believe it would be a digital camera? Also what size prints you're looking at?