Mixed martial arts (also known as MMA), is a hybrid combat sport incorporating techniques from boxing, wrestling, judo, jujitsu, karate, Muay Thai (Thai boxing), and other disciplines. (according to Wikipedia and Britannica). MMA is generally a full-contact sport so you need to accept the fact that there will be contact and possibly injuries are part of your training.
Karate is one of the World’s most popular martial arts. Karate was developed in Okinawa, a small Island in Japan in the late 1800s, and early 1900s. Karate is a highly efficient system of self-defence, with an enormous depth of knowledge developed over many decades. Karate is an excellent form of exercise both physically and mentally, that promotes a great number of life skills and values. It is often referred to as "the thinking man's/person's Martial Art".
Unlike many martial arts, the size and strength of a karate practitioner are unimportant as students are taught to use their entire body when delivering strikes (to generate more power). Students are trained to deliver sharp blows with various weapons of the body towards pressure-sensitive areas of an opponent. For these reasons, Karate has become very popular around the world with women and children in particular.
While both styles teach efficient personal combat skills, karate does however (arguably) emphasise self-control more and promotes the awareness to avoid conflict at all costs. Combat should only be used as a last resort.  Mixed martial arts is more of a 'sport', whereas Karate is more of an 'Art'.
In short, both Mixed Martial Arts (MMA) and Karate have good and bad instructors and are very efficient in their own right. So it comes down to your personal preference.