I speak formal. I dip in and out of formal, and also in and out of ebonics because it's how I was raised. I know how to speak formal when the situation calls for it. But when I'm talking to us, to other black people, I speak ebonics (within reason). I'm not going out of my way to say slang that doesn't come natural to me.
With that being said, if I'm interviewing or talking to folks at my tech job, I talk formal... but I still sound like a brotha.
I'm not about to go out of my way to take the bass out of my voice like David Banner did on CNN that time, or make my voice sound nasally to sound white. I'm a black man. Either they good with that or they not, and I ain't about to try to appease them white folks.