Pro's and con's of both in no real order:
Currently less quality channels on Freesat.
I would argue that Sky's EPG is far superior to any of the Freeview Box's.
If you would like to buy a PVR then you would have to go Sky+ with Freesat at a cost of £10pm. Lots of models available on Freeview.
You could sign up to Sky for an entire year, receiving free equipment and install, then cancel after 12months. Overall cost will be around the same as a Freesat or Aerial install.
Easier and cheaper to distribute the aerial feed around the house so all TV's can receive Freeview.
Note:
There are currently LOTS of Aerial "cowboys" around, they do shoddy work and have no real idea what they are doing. If possible chat to the guy and find out which transmitter he is going to align you with and ask if he uses a meter. Most cowboys are so stupid they have no idea of the transmitters and don't even own a meter, they probably would have no idea how to use it anyway!
Any more questions then fire them away.
EDIT - Just noticed you said you already have all of the Sky equipment, do you not have any sort of viewing card?