-You can trade any pokemon into the game at any time without getting a national dex
-Easier access to Groudon/Kyogre if you want to use them earlier
-Certain pokemon such as Zangoose and Medicham can't be obtained in Emerald
-New improved breeding system
-Some pokemons' abilities have new outside-of-battle uses
-You can get a 2nd gen starter (Chikorita/Cyndaquil/Totodile), although this will require a lot of trading.
-More of Team Aqua/Magma
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.
Those are the Pokemon you'll be missing out on if you choose Emerald. You also get to choose your Lati. Cloning if you're into that. If you're a breeder, there is the Everstone addition that was already mentioned as well as Flame Body and Magma Armor. Many other ability addtions, too. The Battle Frontier gives you a bit more to do after the Elite Four and some cool items, I'm pretty sure you can't get in Ruby or Sapphire.
Emerald for me. I have it. You can rebattle the gym leaders. The Battle tower. And choose a Lati after beating the game instead of getting an eon ticket with your version. And much more... A nice improvement of Ruby and Sapphire.
Definitely Emerald, because of easier breeding, all three of the Hoenn legendaries, and most of all BATTLE FRONTIER. You'll be missing out on something great if you buy Ruby or Sapphire.
Oh, and the Pokemon move. Kinda cute.
The official sell price is $35. Individual people selling their games doesn't count because they essentially sell anything for any price. Not to mention the Internet isn't the best palce for buying used Pokemon games. I think Amazon has a decent rep with it, but chances are you'll end up with a pirated copy on eBay.