mk9 by default since it actually had a playable pc version where i've spent actual time playing it both off and online
but considering what i got on release...disregarding the most important part which is the gameplay...
graphics
overal design: mk9. simply because i was more impressed by it when it came out in 2011, rather than the plastic-looking mkx. it felt colorful, but not childish. the ui design alone beats the snot out of the bland mkx style. the over the top damage system was hilarious.
fidelity: mkx has the edge and it's not a brainer. 2014 vs 2011. it did surprise me though how good the mk9 textures were, considering i ripped most of it.
with the correct lighting it looks good even this day.
character models: lel. bodybuilders heaven in both games. in mkx, females are actually clothed sometimes, but they look like midgets. at least mk9 females had raging abs and these things called muscles...the only plus that mkx has is the fact that males have height variations. that's it. lack of cyborgs is a big minus.
btw expect seeing females getting some weight in the upcoming games, seeing as boon supported that trash "badass realistic females" article (sjws here we gooo)
sound: mkx has some rocking tunes, but so did mk9. the sound effects are good in both games. tie.
in a nutshell: when i paid £25 or something for mk9, i got a playable video game. when i paid £50 or something for mkx, i got a bag of feces. which one would you choose, really?