>>>They both seem to have the same channels
Oh no they don't!
Freesat has far more channels but you probably wouldn't want the obscure additional ones anyway. Freeview carries the free UKTV channels (Dave, Yesterday, Really and Drama) which aren't available on Freesat.
Your options are as follows:
1. Simply keep your Sky box (with the old card in it) but allow the subscription to run out. You'll still be able to get all of these channels:
http://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
2. Replace your Sky box with a Freesat box. You'll then get these channels:
http://www.freesat.co.uk/what-you-get/our-channels/
(In practice, the only real advantage over Option 1 is that you gain Freesat's electronic programme guide, which would otherwise be missing, plus access to to some 'red button' extras).
3. Switch to Freeview, giving you these channels:
http://www.freeview.co.uk/whats-on/channels
As a lover of Dave and Yesterday my own choice has to be Freeview.
Chris