Realize, most importantly, we're not talking about self defense, but fighting. I don't consider any fighting instruction without a detailed use-of-force lesson to be a legitimate for self defense. Anyone can teach you how to fight well. But attention must be paid to the fact that various techniques will be viewed differently in court and you really haven't defended yourself if you kill someone with the "five point palm exploding heart technique" and end up as someones bitch for 7 years on manslaughter charges.
If you're looking for effective street fighting skills, you probably don't want to go just with Boxing or BJJ (depending on who's teaching the BJJ). Boxing teaches you to punch hard, but you're gonna need someone to work with you on kicks and clinch fighting (the kind of stuff that tends to f-up boxers). a lot of BJJ places focus nearly totally on groundfighting. This stuff is way overrated for actual street usage.
A lot of places teach both BJJ and MT. concentrate mainly on your stand up game and keep the ground stuff related to how to stand back up. For the adult classes, make sure the instructors are gonna teach you how to fight dirty, and not just prepare you for an octogon battle.
There's sport MMA and Street MMA. You want to concentrate on the latter. For an example of the latter, look at some of Bas Rutten's work (interestingly enough, he recommends krav maga as a good base for street MMA)
I think a good street MMA program will consist of MT & boxing (for your primary stand up game), BJJ (for getting yourself off the ground) and philipino martial arts (for stick and knife defense)
To get decent fighting skills fast, combatives are the way to go. Krav Maga, Fight Survival Training (FIST). Each of those programs takes about 6 months to master compared to a MMA program which can take years. Combatives is the basics, Martial arts is a PhD.