FreeView is what you know and love through you pointy thing on top of the house, you'll have HD channels on it now or soon, you will need to buy a TV capable of it or a little box. HD does require a new box if you're bothered.
FreeSat is the dish one, many more channels but at the moment you don't get things like Dave and some Sky one's
You do get 4 HD channels currently (BBC 1, BBCHD, ITV HD and C4 HD) reception and picture quality for me is superior and seeing FreeView HD while on holiday (we dont' have it here yet) FreeSat is the way to go

Things are added and iPlayer and soon ITV player is on it. I also thinkFreeView HD might have this now (need broadband connection)
Of course you'll probably use both like me, as my TV has it all internally.
There are cheaper add-on boxes if you just want to get it going. You just need to move the dish to the correct satallite. Either guess, use dishpointer website to help or some fangled iPhondroid thing. Or £5 sat finder and learn to use it properly
I used Dishpointer in a google amp to get direction then 3 fiddles in the heavy snow and I get 9/10 reception. I bought my dish for about £22 of ebay with cables, multiuser setup bargain, quality cables and same as the Sky dish.
Sky no idea, you get more of everything but you pay for it.