Simple answer GA only. This is because you are unlikely to owe any SC tax*.
But taxes aren't simple. It somewhat depends on your total income. SC starts out with a lower tax rate than GA, but at higher incomes, the SC top rate slightly exceeds GA (6% vs. 4.99%).
Start by asking your employer if they will even withhold SC tax or if they require you to fill out both forms (the SC W-4 and G-4). Many employers will only withhold for the work state.
*The general rule is: your report all your income on your home state return, even the income earned out of state. You file a non-resident state return for the state you worked in and pay tax to that state. Your home state will give you a credit, or partial credit, for what you paid the non-resident state. You will have to file a non resident GA state return and pay GA tax on the income earned there.. You will also file a SC full year resident return and calculate tax on ALL your income. SC will give you a credit, or partial credit, for the tax you pay GA. So, there will be little or no double taxation (so SC withholding should not be needed). But you have the cost and hassle of filing two state returns. Do the nonresident state return first. Doing the returns in the correct sequence insures that your SC return will include the credit for the tax you pay to GA.
