Yea its worth it. If you're saving for Sun/Moon, you can buy ORAS beat the game and use it to set up shop. With a bit of consistency in selling and gathering credits, you will make more than enough to buy Sun and Moon.
Watch out with that one. Not every person has the hardware required to be a seller, and if they're using external hardware, it's incredibly detectable and will make their product look bad. Also, breeders make almost nothing as buyers are looking for that instant gratification that can only come with a quick gen.
On topic:
While it is certainly nice to play ORAS, keep in mind that if you've played the gen 3 games this version stays almost 75% true with its source material. Bits of the story have been retconned in to include mega and primal evolutions, and then the post-game content delta episode brings in a background story on mega stones. So, if you can stand missing those new things, then ORAS is exactly like R/S/E. The music is updated, the sprites are obviously updated, and there are 5-6 new or improved features that really stand out.
However, it's not a core series game and if you're just trying to play those I would get sun / moon first, then reach for ORAS if I have the extra money. You could probably resell ORAS for about 25-30 dollars when you're done with it.
TLDR: Yes if you can shill the extra money and resell it by December, otherwise no.