They aren't the same. Normally in battles you can only mega evolve one pokemon. But in ORAS, you can Primal Reverse Kyogre/Groudon and still Mega evolve another pokemon. It's fun to have triple battles were both primal groudon and primal kyogre are out and having a mega rayquaza in between. Also Primal Reversion happens right when you send out the Kyogre/Groudon instead of when you want it to, unlike Mega Evolution