Digital signals through an aerial like for Freeview are currently being sent out at a "lower" strength so they dont interfere with the analogue signals.
When analogue is eventually switched off they will boost the strength of the digital signal so you may not have so much of a problem in the future.
Sky of course comes through a dish from a satellite up in space, so you cannot compare the signal through an earial to that through a dish.
But if you are under the Sky satellite footprint (over the UK) and your dish is lined up correctly then you should get a decent signal from Sky.
You can get Sky installed as a one off cost with no monthly payment with their Freesat service. This provides about 200 channels.
http://www.freesatfromsky.co.uk/
Dont confuse this with the Freesat service from the BBC / ITV which also uses a dish and a Box, but has no link with Sky.
http://www.freesat.co.uk/