I never heard of an ITP rating. What is it?
Whether you call it a relief pilot, SO, or restricted FO, the job is likely the same: sit in the seat and monitor the gauges while in level cruise. You might get a "big event" and do an altitude change or route modification via the FMS and autopilot. You will get NO manual handling of the controls, and NO takeoffs or landings. You may get "plenty of hours," but they will be relatively useless as flying experience.
In the worst case scenario, you may get yourself into a critical situation like those poor sods on AF447, and be unable to figure out how to get out of it.
TRAINING and CERTIFICATION differences may determine when and how you progress from a low-paying RP/SO job to a higher-paying FO position, or if you ever get to try for a Captain position. Without PILOT IN COMMAND time, you can't an ATPL. Without an ATPL, you won't get to the Captain's seat, and in many [US] airlines you won't even get an FO position. The US doesn't recognize the MPL (Multi-crew Pilot License), and there is an uncertain (at best) upgrade path from the MPL in those countries that accept it.