I lived in the Atlanta area for 13 years and I vote for straight through. Get in the second lane from the left side of I75 as you get close to the outer fringes of Atlanta and just stay there. The right lanes are always becoming exit only lanes if you aren't careful. As was said earlier, avoid the worst rush times of early morning and mid-afternoon. As you get close to the junction of I75 and I85 there will be two exits to the left, but you will be fine in the lane you are in. After joining up with I85 you should move over to that second land from the left as soon as you can and just stay there until you are well south of town.
I285 around Atlanta in either direction is much longer and the traffic is as bad or worse than going straight through. Trucks as prohibited inside the I285 perimeter unless they have paperwork showing they are loading or unloading inside, so you will see most all truck traffic on I285.