Street Fighter was more fun to play, but whenever I went to the arcade in the early 90s I was always drawn to watching others play Mortal Kombat. It was like watching an actual Kung Fu movie where players could control the characters.
Street Fighter, MK was cool though. I'd basically go back and forth between the two. I probably put the most money/time into SF2s various editions.
Heart, Head, and Spirit says Street Fighter. Cool factor says Mortal Kombat. It was alot harder to play on MK, Mk2, and MK3 machines, much more traffic. Legacy ... Street Fighter.
As far as depth the recent installments of MK and SF both have it. Game can be played both casual and competitively. They're still played at competitions like EVO. Popularity wise, probably SF is still king. MK still popular in the states. MK has better guest characters, from DC, to horror/action films (Freddy/Jason/Leatherface/Terminator/Alien/Predator). MK's gore was big in the 90's is considered rather tame now. Graphics and art style I personally prefer MK. Sound effects definitely MK. Story and lore is definitely MK. SF 2 /3 sprites are a classic, alpha series had the anime style nice as well. Never quite liked the water color style of SF 4 and 5. They go more cartoon caricature, which is fine but things like how all the characters' hands are disproportionately large is an eye sore. Both fun games, and they haven't fallen off per se.
It depends on the specific game. MK2 and MK2 were cool but most of the rest were hot garbage. Street fighter requires more skill and is more fun for those that are more competitive. MK was more about the characters and the concept. It's only there for nostalgia now.
Street Fighter II is better than the original MK. But overall the MK franchise is better. Tekken 2 and 3 are my favorite fighting games though.
Street Fighter II. First, it was the only game I'd get to come out to play my freshman year in college rather than going to class. Second, it was one of the last games on the SNES that we could get a ton of friends together and take turns playing. I was always Ryu and I was a total one trick pony. But I did it because I was so good at it, that it would piss my friend's off to no end. Those were fun times. Brief shout out to the OG SF. Yie Ar Kung Fu.