Grady is good. Busy, covers all primary 911 for the City of Atlanta (which is actually quite big) and apparently some report counties in south Georgia too. SSM, all ALS trucks with mostly Intermediate/Medic I believe. Has about 40 ALS trucks, 4 BLS trucks for IFT, and a special operations division. Of course in city limits, you get a old variety of calls that'll keep you busy depending on where you are. Short transport times for the most part.
In the suburbs, you have Rural/Metro running 911 in North Fulton county. It is a more affluent region, with more SNF type calls. Little to no trauma or such. R/M is primary with fire response/back-up (each city has its own BLS truck or chase car or something). Also SSM.
Gwinnett County is fire based EMS and I believe that they have opened the first in-house paramedic school in Georgia. I think you have to be a FF/medic to get hired on. Decent county with good mix of call types.
Dekalb County is run by AMR. Dekalb is known for being the craphole of the metro area (aside from certain parts of downtown) so probably have a lot more traumas and "exciting" stuff there. Also SSM I believe, but they have a fairly large fleet.
MetroAtlanta, Central, and Rural/Metro are some of the major IFT players in the region.