I have trained many cadet pilots to become First officers and then much later Captains, also experienced GA/ commuter pilots to do the same. They both do their jobs very well in the end, the end standards are supposed to be the same after all. Obviously the main difference is experience. The guy with more experience is likely to be able to think outside the box and handle more complex situations earlier in his career than someone with less experience. That is why most airlines have a min experience requirement for command and not just seniority in the company. Ideally then, cadet pilots will spend more time in the right seat gaining that experience, than an already experienced ex-ga pilot.
eg. A 3 year GA pilot will have had a lot of experience dealing with a vast array of issues applicable to keeping himself safe, and legal, often against the wishes of his boss or clients.
A 3 year cadet will have been some what protected from the above issues by the airline system and his Captain. However he/she will have been watching how these problems are resolved by others around them while gaining experience with the aircraft and routes they will probably later have to fly as Captain.
Both pilots will have to learn the skills and experience that the other has before they will be ready for a command. Until that time they both have strengths and weaknesses which is why they will be in the right seat flying with a Captain.