Red and Green were released 8 years prior prior to FireRed and LeafGreen. It has been 8 years since Ruby and Sapphire were released.
However, in my opinion, the major difference between the two is that the Pokemon from Ruby and Sapphire can still be traded up to the newer games, unlike Red, Blue, Green, Silver, Gold, and Crystal. This is not to say that Nintendo will not release remakes of the two (which would be a welcome change, since the music and graphics in Gen III were subpar, at best, even if it did revolutionize competitive play), and if they do, I would welcome it, considering how HeartGold and SoulSilver were amazing games. However, it is not as sure-fire as FireRed and LeafGreen.