Best features IMO of CS over 7.
The higlight/shadows tool: reduces highlights, brighten shadows to improve your photos. VERY USEFUL. I use it every day.
The Colour Match feature: say you had two photos of a person you wanted side by side on a web page but the colour of one of the photos was slighly different from the other, just use the colour matcher and the desired photo will match the colours of the other.
The Filter Gallery: You can now preview all filters from one filter menu/window and you can stack filter effects on top of each other in their own filter layers so that they have an interactive effect on each other.
Yuendos, I'd go for CS if I was you, unless you want to scan US $20 bills. Then go for 7.