Cognos is just a BI tool ,very simple data analysis software(or just do some summary statistic).
SAS is an advanced data analysis software ,which can do many stat/or/ets models (which is very complicatged).
and SAS also have many product like BI EBI DI ...... as Cognos.
You can take SAS as SPSS (another IBM statistic tool).
One of my favorite about SAS is its data cleaning power.