The A80's better focusing system and swivel LCD are really good things. 4MP is crisp and sufficient for most tasks. The S50 has only one advantage, actually two, one is it has RAW format, and second, it is black
. Overall the A80 seems to be a better option for most people. I've purchased two already and find them to be very useful cameras.
This comparison has been beaten to death on these forums for the past year, so do a search and you will find comprehensive comparisons of the two models and hundreds of responses to why a particular choice is better.
Last week Office Depot had a fluke sale and were giving away A80's for $180 + tax, so if you stumble upon a deal like that, A80 is the way to go.
Before I buy, I would read everything I can and then actually try out both models. I like both cameras, but the small niceties of the A80 such as the swivel LCD make it indispensable. 8)