Rn MK 11 is at the top of the list just as you said it has better graphics and awesome finisher moves, the thing is MK is a game that often gets forgotten, while tekken has fans playing it over a decade, and devs are like more into it making it better and pushing newer versions quicker, the both are good games, my opinion is that buying tekken maybe will give you more playable time, meanwhile MK will get boring at some point.
This is only my honest opinion, please if i'm wrong someone correct me, and don't buy any of those yet, wait until some other opinions from other members only after a few opinions choose then which one to buy.