If you are driving through Atlanta on a non-holiday weekday, you should consider morning rush hour to be 6AM until 9:00 or 9:30AM; afternoon rush hour can begin as early as 3:00PM and last until 6:30 or 7:00PM. Saturdays and Sundays can be very good days to drive through, but you need to check out some sports schedules
Driving straight through downtown Atlanta on I75 saves quite a few miles over going around the city on I285. If there are no traffic problems, downtown is no more difficult to drive than going around the city. But you need to strategize the day and time of day. Alternatively, you can bypass Atlanta by taking I285 around the west side, and then pick up I75 again on the south side.
Maybe your GPS will give you all the info you need (I don't know?), but if you have a Smartphone, you should check the Georgia Navigator website well before you get to I285 on the north side. It will tell you about any traffic accidents/incidents as well as congested areas. You can also listen to traffic reports on AM750 or FM95.5. I'd suggest that you start looking at and listening to this info probably around Cartersville. That will give you sufficient time to choose your route --- I75 through the city or I285 around the city.
Also --- depending on the time of year that you are coming --- you should check the schedules for Atlanta Braves, Atlanta Falcons, and Georgia Tech football and basketball. Driving through town DURING a game is just fine. Hitting town when fans are arriving or departing is not.
See, I don't believe that I285 is automatically a better route than going through the city. Even it can be problematic if there is an accident. I'd suggest that you decide ahead of time which route you "think" you want to take --- but you may want to be prepared to go the other way, depending on traffic reports as you approach the city.
Although traffic can be unpredictable, and you may need some patience, driving through Atlanta doesn't have to be a horrible experience. Just make use of good sources of information ahead of time.