BJJ is a great fighting form, not only does it work to control people and as someone else stated present a fighting form most people do not understand, it takes what natural instinct is and uses it against the other guy.
The only problem is it works for great for 1 on 1 fights, and it gives a chance to anyone watching to step in and put a boot in your neck.
Krav "TO ME" is the best of the "self defense" martial arts, it takes the idea that the best way to end a fight is fast. The longer you are engaged in a confrontation the more chances for the other person to score a lucky punch, pull a weapon or have friends show up. So the idea is to end the fight in less than 10 seconds, Krav goes for quick simple and brutal fight finishes.
So my suggestion is if you go for a second art find a good Krav Maga school.
That being said I train Muay Thai ;)