Diana,
I've used the free PAF and Legacy in the past but was persuaded to buy Family Historian when I visited the Fam. History Fair in London in April, after seeing various demos. I think the main feature which makes it different from other programmes is that you can produce colourful and, if you wish, very big family trees. It's British, which may suit you if you don't have any American relatives and there seems to masses you can do with it when you learn how! There are very helpful people who answer queries on-line too, but it is very easy to use, so the queries seem to be about more complicated matters! It has become very popular, also being used by the BBC in its Fam History programmes.
The web-site is
http://www.family-historian.co.uk/

if you want to have a look. Good luck! Jo