A professional roofer or roof supply can give you the cost breakdown and pros and cons for each material for your Atlanta area.

I understand your hesitation! Took us years to make up our mind. We had cedar shingles or a neo-mansard second floor and needed a non-cedar alternative. We went with GAF Timberline HD in Pewter Grey. The house is still in the middle of an exterior reno (new siding to come) but you can get an idea of what it looks like from this (not so great) photo:

Synthetic shake and slate options are great for mansard applications duch as Enviroshake or Inspire. There are also algae resistant shingles available.

I'd be interested to hear what material you chose in the end. I had my mansard roof done in Certainteed Landmark architectural shingles 10 years ago and what a huge mistake! Every year I have some shingles fall off which started either the year of or the year after the installation. The roofer blames the manufacturer and vice versa. Certainteed has denied the claim but I've read about many customers having problems (even with the fibreglass asphalt shingles). The roofing company comes every year to replace the fallen shingles but it is very frustrating and unacceptable to me. Here is a photo of the front of my house currently with 6 shingles fallen (although not all are visible in this photo). I'm not sure what to do anymore.

Steel shingles.