my self got both.
"completing the National Dex." that will need more then one game, or one hell of a lot of trading on GTS and with friends.
when Ruby and Sapphire 1st came out, the main game of the 2 i played the most was Ruby, this time with OR and AS i playing AS as the main game and OR as the side game.
i normal have 1 game from each Gen to play as the main,
for G3 as i said that was Ruby,
for G4 that was Pearl
G5 is White,
and as i have yet any real play time in to X or Y, it look like my G6 main game may be AS. for now any how.
o and any one interested G2 main game for me was Gold. and G1 was Red, (never owned Blue, that is the only one i have not owned.)
if you can only get one game toss a coin, as that the best way to pick, one game is not better then the other.