The kings of fighting games going at it once more.

NOTE: When I say Street Fighter, I only mean Street Fighter, not Final Fight or Slammasters or Rival Schools or other fighting games that involve some SF characters.

Plotline & Characters
I think as far as plots go, Mortal Kombat is better! but Street Fighter has an interesting plotline too, although on part 3 they slacked off with half-assed plots (one fighter in 3 wanted to win his car back...his car!) But they've got some interesting characters with cool designs, and the characters seem more flexible with their personalities rather than the Mortal Kombat characters do. What's cool is that the Street Fighter EX series are AU, they don't take place along with 1, 2, the Alpha series, and 3. So, that is also really interesting.

Gameplay
Mortal Kombat has much more interesting features, but the cool thing about Street Fighter is not one character fights the same as the other. However, Mortal Kombat eradicated this problem in MKDA, which makes Mortal Kombat the better fighting game.

IMO, they're pretty evenly based. But if the two games ever had a fight, Mortal Kombat would win and not just cause of fatalities.

We already know the Mortal Kombat storyline, so for those who want some background info on the Street Fighter series plots:

SF1
The greatest fighter in all of Thailand, Sagat, wants to expand his name across the world, not just as king of Muay Thai. So, he decides to hold a tournament for all fighters to enter.
THE RESULTS: Ryu defeats Sagat and is claimed to be the best fighter ever.

SF Alpha series (1, 2, 3)
The Alpha series gives background to Street Fighter 2. It tells how rivalries between characters are formed, how some of them joined M. Bison, or went against him. Also gives the story of Ryu's battles with Akuma and the Darkside of himself, and how Ken fits into it. It also follows M. Bison's quest to brainwash fighters to use them as part of his quest for world domination for Shadowloo.
THE RESULT OF ALL 3 GAMES IN THE END: M. Bison's empire is shattered (not destroyed, though) when Chun-Li teams up with Guile and Charlie. Bison appears to be dead, but he'll be back. Also, Ryu manages to beat back Akuma, but he knows Akuma will be back for him.

SF2
M. Bison wants to build his empire up again, so he decides to hold a tournament and invite the best fighters. However, some forces should never have been invited.
THE RESULT: Either Chun-Li or Guile beat M. Bison and finally put Shadowloo down. However, M. Bison is destroyed by Akuma once and for all, when Akuma decides to test his might against a fighter like him.

SF3
This new empire known as The Illuminati has been illegally doing things to ensure that they will save the Earth from the Apocalypse. Once they save it, they will lead the Earth to a new era. In order to choose which fighters they will brainwash and experiment on in order to accomplish this task, they hold a tournament to decide who the lucky fighters will be.
THE RESULT: well, when SF4 or whatever comes out, we'll know, but according to Capcom, Gill and the Illuminati have their eyes on Alex.

^^^^All of that isn't made up. It's official canon confirmed by Capcom.