I have owned them both. The 1SS interior is nicer than the base model GT. The Premium GT however is much nicer than the 2SS/RS.
Both cars have their highs and lows but I will say the Mustang has my nod atm. Hence the reason I traded the Camaro for one. My decision absolutely takes nothing away from the Camaro. It is a fantastic ride as well and one of the best I have had the pleasure to own.