MMA because you'll simply practice more styles in on lesson. For example, say you went to a Brazilian Jiu-Jitsu lesson on Monday, you'd practice grappling, and then went to Karate on Tuesday, you'd practice striking. But if you went to a MMA lesson you'd do them both. Also, going to a MMA class is better for learning to combine grappling and striking, while if you go to the separate Brazilian Jiu-Jitsu and Karate lessons, you wouldn't be using them together so you would not know ho to use them together well.
Hope I've helped.