First of all, if you are already a Brigand, it is too late to betray. You have to make up your mind to betray by level 15 i'm pretty sure. I think the information is in the book that comes with the game. I am confident that if you have made your secondary class selection it is too late.
All I can say about my experience with the betrayal quest is that it is totally not worth the trouble!
I did it in about February of 05 because I wanted a Freeport high elf citizen, as a jewlcrafter. I believed the racial jewlcrafting bonus would be significant.
I am not sure if things about the betrayal quest have changed since then, but i definately wouldn't reccomend it.