Its not so much Freesat vs Sky, its Freesat, Freesat HD or Freesat HD+ (hard drive recording) vs, Sky box, Sky+ box (sub required for hard drive recording), or Sky HD (sub required for hard drive recording).
Not much point in going for a standard Freesat bx - go for a sky box with a freesatfromsky card (non subscription card)- 10or so more channels.
A Freesat HD box gets BBC1HD, BBC HD and ITV1 HD.
A Sky HD box with no viewing card gets the same.
A Sky HD box with a viewing card ( a freesatfromsky card (non subscription card) - gets in addition 4HD and Five HD.
A Freesat HD+ box allow you to record to hard drive for free.
A Sky+ or Sky HD+ box needs a subscription to record to the hard drive.
Not all channels are onthe freesat EPG, but you have to put the box into NONfreesat mode and scan for other channels. (Sky news being one example of a channel not on Freesat, but in Nonfreesat!)
So at the end of the day it depends:
do you want Fiver, Five USA, Sky3, - sky card / sky box
HD? which HD channels - just BBC and ITV - either freesat hd or sky hd, Four HD and Five HD - sky hd box and sky card
recording - free: freesat or a generic receiver, or if you have a sky sub: sky+ or skyhd+