A Sky box without a viewing card will get most FSFS channels but you can't regionalize it properly because Sky sets the region on the viewing card rather than in the box itself. A Freesat box sets its region via your postcode and has no need of a card.
Be wary of Sky boxes currently for sale on fleabay. There was a major programme to replace all viewing cards last year and it is likely that many boxes are now on the market whose owners didn't see the point of paying for the new card. I would say it is safe to assume if you buy a box on ebay you will also have to plan to buy a new viewing card from Sky.
There are few differences in the channel line-up between Freesat and FSFS but nothing very significant. All the main entertainment channels are on both and most of the very obscure channels that are missing from the Freesat EPG can still be tuned manually if you desperately want them. Have a look here:
http://www.freesat.co.uk/what-you-get/our-channels
and here:
http://www.sky.com/shop/freesat/home/what-can-i-watch/