I'd definitely recommend Emerald.
1. It allows you to catch both Groudon and Kyogre (as opposed to just one of these with R/S) & Rayquaza.
2. You get access to the 7 attractions of the battle frontier. (In R/S you only get the battle tower).
3. Once you defeat the elite 4, a new area opens up in the safari zone that allows you to catch 2nd gen pokemon (nothing new in R/S).
4. Once you beat the game, you gain access to place called desert underpass, where you can not only get the fossil pokemon you didn't choose earlier, but you can combat and catch dittos. You can of course you dittos for breeding, but I use them frequently for EV Training. Simply lead with the pokemon you wish to EV Train, then switch to the pokemon whose EV values you wish to obtain. Ditto will turn into that pokemon, and, once defeated, will give Exp. and EVs as though it was the transformed pokemon (Desert Underpass isn't available in R/S).
5. The game is a pretty green color.