Seamless or sectional
Seamless aluminum gutters are formed on site: the crew brings a roll-forming machine and a coil of flat aluminum, and runs each length of gutter to the exact length of the wall. The only joints are at corners and end caps.
Sectional gutters come in fixed lengths and are joined along the run. They are what a homeowner can buy at a store and hang without a machine; each joint is one more place sealant can fail.
5-inch or 6-inch
Gutters are sized for the hardest few minutes of rain, not the yearly total. The method in SMACNA's Architectural Sheet Metal Manual uses the five-minute rainfall expected once in 10 or once in 100 years, and points to NOAA's data server for the local figure. In metro Atlanta that five-minute, ten-year rain runs at about 7.7 to 8.2 inches an hour, depending on the place (NOAA Atlas 14). A long roof section, a steep roof, or a valley that dumps two roof planes into one corner is where a 6-inch gutter and larger downspouts earn their keep.
The gutter and downspout calculator uses your city's figure to show the flow off a roof section and how many downspouts it needs.
Downspouts
More downspouts, or larger ones, often fix more than a bigger gutter would. By the rule in the Copper Development Association's handbook, one square inch of downspout drains 1,200 square feet of roof at a rain of one inch an hour, and proportionally less as the rain gets harder. A 3-by-4-inch downspout has twice the opening of a 2-by-3.
The installation visit
The old gutters come down, the fascia is checked and repaired where it is soft, hangers go in at the spacing the run needs, and the gutters are pitched toward the downspouts. A typical house is one visit. Ask what happens to the old gutters and who hauls them.
Before you sign
Get the material and thickness, the size, the hanger type and spacing, the number and size of downspouts, and the price in writing. Hiring a gutter contractor in Georgia covers what the state does and does not license.