>>58271499I say this pretty much all of the time to people but when the credits roll, there are many different cases where this is not the end of the game and Pokemon is one of them. When the credits roll, the only thing that actually happens is that you have added a star to your trainer card, and there is more than one star to add (four in total, actually). "Post-game" as a term is actually already pretty disingenuous because of this and I've never liked that term.
Becoming Champion is logically the best place to put the credits because it's pretty much at the end of the scenario (and I can't argue with the giant THE END screen at the end of the credits) but in addition to becoming champion, your efforts are also awarded in pretty much the same way by the game in the following ways:
>Become a master at all contests in the game>Win again 50 trainers in a row at the Battle Tower>Complete the Hoenn PokedexThe only one of these not locked to postgame stuff is the contest mastery. Technically you can complete the dex as well, but you may not be able to if you are emulating these due to version exclusives, so for you will would just have to catch all you can in the version that you pick, which does require beating the Elite Four.
But to answer your question, playing Ruby/Sapphire/Emerald is pretty much mostly the same if you ignore the postgame, the only real addition comes in the form of having noticeably more Double Battles in Emerald, which if you haven't played a Pokemon game at all, definitely shouldn't be the start for you (not because they are hard but because most of the series is primarily single battles).
