As a matter of fact, BJJ was mostly developped by Helio Gracie, who has really skinny and weak. He developped that sport for him smaller and weaker guys to take on bigger and stronger guys.
So just keep in mind that what your friends are telling you isn't completetly stupid.
That being said, just go for what you like the most. After all, I am pretty sure this is going to be a hobby and you won't be competing at a professional level anytime soon.
Just have fun