Sky HD only seems marginally better than standard because it is broadcast in 720p not 1080I or P, as far as I am aware freesat is the same so effectivly 1/2HD.
But if you us a good dolby digital amp the sound quality is far superior to std TV, I use a dolby digital 6.1 system all the time.
IF you want the full benefit from your HD TV you want a BLUE RAY Player, that is defo like the difference between B&W and colour, coupled with a SS amp that can supply THX Sound the experience is far better than any trip to the cinema, and whats more you can press pause to go out and get another pint from the garage.
The Main benefit of freesat HD over the others is ITV HD is on freesat but not on any of the others, you will also be able to recive channels for free that sky don't allow you to view.
I was looking at Freesat boxes, but think the way to go is get a decent make INTERNET enabled LCD with Freesat built in, then you can view all the new internet TV services such as BBCI, CHannel 4OD, ITV online, YOUTUBE, etc etc etc, and also watch freesat services, Thats the route I am looking at and cancelling my sky HD subscription.