I've been to both. I stayed in Mt. Pleasant east of downtown Charleston. Driving in, I missed the road to the KOA in Mt. Pleasant and crossed the bridge into downtown Charleston driving the 'C' and pulling a toad at around 5 PM. Traffic was a nightmare and, pulling a toad, it took a long time to find a place to turn around without backing up. Once I got out, I never went back in. I visited Patriot's Point and toured the USS Yorktown, a WWII aircraft carrier like I served on. It looked the same and smelled the same. Those old carriers burned black oil for fuel and the smell never goes away. But there aren't many left that haven't been turned into razor blades.
Savannah's claim to fame is Ghosts and there are several tours. I didn't take a tour, but did drive the toad around and checked out some beautiful antebellum homes in the area. Both cities are worth a visit.