If you want competitive fighting, SF is a no brainer. MKX's system is a lot better than any other MK game in the past, but among the hardcore FGC, Street Fighter reigns supreme and you will get much better competition online and at tournament events. If you're not as interested in tournament play and are just wanting to have fun beating the bloody shit out of people, then choose whichever one you think looks better to you- in this case Mortal Kombat is probably what you're looking for. Check out some YouTube videos of each and see what you're more into. I personally have always liked the depth of Street Fighter games over Mortal Kombat, but both are fun.