Knowadays, if you watch MMA you'll learn some rudementary BJJ skills. Definitely take the opportunity to learn BJJ, because it's really important to know how to use the submissions, but it's even more important to know what submissions an opponent is going to be trying to use on you.
Sambo is a better style for modern MMA, in my opinion, because you learn takedowns that most opponents aren't used to dealing with and you learn submissions that some opponents have never seen before. It's one of the reasons why Andrei Arlovski was able to submit Tim Sylvia so quickly while Jeff Monson couldn't. When you have submissions an opponent hasn't seen before, you're way more likely to land them.
If you are looking at a career in MMA, definitely train in Sambo full time, but go to some BJJ classes and watch tape of BJJ fighters so that you see where they attack from most of the time and what the signs that they are setting up a submission are.
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Barnett - Toquinho -Werdum - "Nurmie"
Z. Gurgel - Morango - Rocha - Tiequan