Get both Emerald and Fire Red/Leaf Green. In Emerald you can obtain the most legendary's throughout the story, including the trio (Rayquaza, Groudon, Kyogre). The layout of the region is nice as well and I think you'd really enjoy the gameplay. Emerald also allows you to fight 2 evil groups (Magma and Aqua) where as in Sapphire you get Aqua and Ruby you get Magma, but in Emerald their plot behind it is so epic. So Emerald is definitely worth looking into.
As for getting Fire Red/Leaf Green, it's really up to you on which one you choose if any. If you're looking to get FR/LG then look at the Pokemon exclusives to help you determine. I know a few of the exclusives are Magmar, Vulpix, Pinsir, and Staryu for LG and Electabuzz, Scyther Shellder, and Growlithe for FR, personally I like Fire REd. If you played the original BRY then I'd recommend this game for nostalgic purposes and to see how the games have been updated with a few new features.