I think to say that the originals hold up would be the understatement of the century. These games are classics. Emerald in particular is a marvelous game that offers a very different experience to Oras.
First for the good about Oras because the originals and the remakes are both among my top 3 favorite games. Oras had a beautiful ost-- give a listen to the sky pillar or the overhaul for the aqua/magma leader theme. Next I like how it expanded the role of the lati twins, introduced soaring as a feature, added dramatic and creative new story content post game in the delta episode, introduced new characters and expanded on existing characters-- in particular I like that they took an already well-liked champion in Steven Stone and made him I feel the definitive champion by developing your friendship with him even further. The secret bases, one of the best loved features from the originals also had some nice updates with a lovely expansion of furniture options and the real potential to have a Pokemon gym. I loved the additional mega evolutions that were introduced in Oras and think that they found a way to take some of the best mechanics from the Kalos region and incorporate them in a relevant way to Hoenn, a way that expanded on what was already strong about Hoenn, its rich sense of history like Sinnoh and Unova. I was also overjoyed to see Poke Amie return. Would have been nice if trainer customization had come back too!
With all that being said there are contributions from the originals games that Oras did not recreate. The originals preserve the game corner and slot machines, which have been censored out by Oras. Emerald also follows in the tradition of the Johto games, allowing you to rematch gym leaders unlike Oras, and has a real sense of closure with a final battle lvl 70+ as we had with Red. These games also had a Safari Zone with a larger variety of Pokemon from other regions like Houndour, Mareep and Miltank compared to Oras. Emerald also opened a new doorway to competitive play by unveiling the elaborate world of the battle frontier. Many fans missed the variety of post game challenges it had to offer and were upset to have it phased out. I was fond of the battle factory in particular and enjoyed how you never knew what Pokemon you would get to make due with.
The stories are also quite different between gen 3 and 6 and the champion's identity can vary as well. You are able to capture both Kyogre and Groudon in Emerald and in my opinion the presence of the entire weather trio at war with one another made the main story even better than Oras and created more interesting dynamics between Aqua and Magma. Though again, I did love the Delta episode as post game story content for Oras.
As others have pointed out the original Ruby Sapphire games are also more of a challenge, Pokemon like feebas and relicanth had lower encounter rates, roaming pokemon still existed, the exp share had not been buffed. berry blending was a mini game you had to do well in if you wanted to get pokemon in good condition for contests. The contests also were more challenging at the base level with opponents using jamming tactics that you would now associate with a master rank in Oras. There were also judges that you needed to take an appeal before. There was more of a spontaneity to game play too with flash sales on TV, visits from special NPCs and Pokemon outbreaks.
Still, I think Oras is best of the 3 Pokemon remakes (though HGSS comes close for me) because it had lots of fresh ideas. Its remaking a region that is very close to my heart, and you're not always going to be able to surpass the original in every capacity. There are valuable contributions from Pokemon Crystal like more detail in regards to Suicune's story and now Celebi's event that make it a formidable game in its own right even after playing HGSS, and there are certainly areas in Pokemon Yellow like having your Pikachu follow you with cute animations and talk to you or including characters from the Anime like Jesse, James and Meowth that you don't get in frlg and still make the originals special.
Both the original RSE games and the 6th gen remakes are amazing. Hoenn is my favorite region for its peaceful and diverse landscapes, elegant Pokemon designs, opportunities for creativity by making your Pokemon a contest star and above all its strong sense of community with characters like Norman, Steven and Wally who you are mentored by or must mentor.
Oras is probably my favorite game overall but I don't think it has to be either or. They are just 2 different games that I would rate 5 stars each.