Atlanta Commute: Which is worse- I-75 or GA-400?

Work is right at I-285 (the perimeter) & Powers Ferry, in the Cumberland area. If we look at homes a little farther north of the city (bigger lots & more for our money), which commute would be worse: coming down I-75 from the Kennesaw/Canton area & getting off at 285, or coming down GA-400 from the Roswell/Dunwoody area (and getting off at 285)? Both seem like parking lots in the morning & evening, but we've never really had to do them & compare. Thanks for your help!