The most reliable data will always be found in an Amadeus engine as this is the system SAS uses and contains live information.
The availability information in other booking engines is not live (however when you try to make a booking it checks it against live data of course).
Some online TA's are having difficulties at the moment because of this, as their booking engine shows availability even though there isn't any... It's apparently a new change from SAS...

So as far as I know (somebody can hopefully confirm or correct me) the ITN tools are Amadeus connected and should show live data. I thought that Etour was as well, but it could be that it is Galileo. In my experience it is much more reliable than the German Aeroplan Galileo though.