I used to do Taekwondo for a number of years, and it helped me out in alot of fights. Both are useful in a streetfighting scenario as when you regularly practice ANY martial art, it becomes second nature, so when someone provides a stimulus (ie throws a punch), you respond lightning quick without having to think.
I would go with Taekwondo as it also includes throws and weapons (at advanced levels) as well as kicks, elbows and punches, but both are useful in a fight against Joe Bloggs. That said, in pro fighting you might wanna take up one of them as well as jui jitsu
depends who's ass you wanna kick!