Well with Qatar you will be a SO for at least 2 years, that means cruise pilots so I guess you can't log any hrs plus you are not allowed to land or take off. I can imagine it can get pretty boring at times. Plus the Salary as SO with Qatar will be less than what a RYR FO makes. Living in the Middle east on a low salary is also not the nicest thing, prices are way up there almost double as compared to countries in southern europe... I know people who lived there and they all said the climate and lifestyle sucks if you are not super rich down there... The main advantage with Qatar though is that you do not have to pay for the typerating plus extras, which is a biggie though...
It's actually not an easy decision...
But you can considder yourself as pretty lucky if you are in the position to have these options...