Not many containers go in RoRo, tends to be more odshaped, outsize ,keep dry (sometimes??)cargos. Perfect for wheeled aircraft, you can put it on a maffe trailer if it is skidded.
If you can get it into a container and onto a container ship then that is usually cheapest for of seafreight- RoRo freight is more expensive .
If you are offered deck cargo,or freight on a container ship, with the aircraft outside of a container (If you are told it can go on a flat rack on a container ship, that is effectively the same thing), avoid it at all costs - it may seem cheap but when it arrives full of salt water it will work out very expensive!