I've been playing both MK and SF since they were first introduced in the arcades. Overall, I've always preferred SF much more as it Game-play is a little deeper but both are great Games and you probably can't go wrong with either.
A side note though, I know the Xbox 360 Controller blows for playing SF, the same is probably true for MK. If you get either you will want to also get a decent quality Fighting Stick to fully enjoy them.
Oh pro tip - invest in the controller that has the dpad pull out or in. Analogs do not cut it in MK where as in SF I could do ok with Analogs. MK a good Dpad is a must. you will find Analogs to be unplayable.
I say MK. but I am a fan of both. each of them has there advantages and disadvantages. But to me MK seems to be the more balanced of the 2, definitely has more players online at the moment and your more likely to find someone around your own skill level
due to being able to see people's wins/losses in each lobby.
Not much spam either unlike SF. and as posted before - you get much more of your money's worth from MK. story mode will take you anywhere from 4 to 6 hours depending on how good of a fighter you are and the story is actually surprisingly good. then
there is the challenge tower has 300 challenges which can be brutally difficult at times especially 250 on up. that took me about 2 weeks to complete minus the 3 days it took me to beat challenge 300. Krypt which unlocks tons of bonus content.
they seemed to have fixed a lot of the online issues as well. KOH rocks but at the same time I really liked SF theater mode
and to answer your questions.
graphics are personal preference, both are great SF has more cartoonish // anime look while MK seems to be more digital
MK I think has more players at the moment, I could be wrong, but your more than likely to find someone near your skill level than on SF
MK wins the sexy females contest hands down. plus fatalities are great
IMO, I think it depends on what you want out of your fighting game. If you enjoy the single player experience, going up the arcade ladder, the game's story, finding secrets, and completing challenges, then MK is the superior game. But if you enjoy the multiplayer
experience and competing online, then SF is the superior choice. The netcode in SF just seems to be a lot more optimized than MK's.