I think they are both nice choices for a ladies’ weekend - I’d make the decision based on ease of getting there / flights from your respective hometowns. Have fun!
I love both Savannah and Charleston. Similar in feel yet different enough to warrant a visit to both. IMO, Charleston has better food but I love walking Savannah’s squares. I would pay attention to the time you are visiting…you wouldn’t find me in Savannah this week (St Pat’s Day) or during SCAD graduation. I wouldn’t want to be near Charleston for Spoleto unless I was going for the event. Both cities have busy weekends that I would avoid if the goal was time with my friend. Easier dining, housing and traffic. Have fun…both are fantastic places!
I agree that you should let practical things help you choose. Don’t forget that there’s an airport in Brunswick, GA too if that helps. I think that Savannah is a bit more laid back and more fun to walk around.
You can’t go wrong with either place. Tybee Island is just off the coast of Savannah if you want to hang out at the beach-it’s a VERY laid back island (sort of the opposite of St. Simons or Jekyll island, which are gorgeous and fancy-schmancy). Kiawah Island is off the coast of Charleston, and is fancy-schmancy, but you can rent a bike and bicycle for miles and miles along its wide beaches. So much fun!