Below are some random thoughts on both.
Charleston:
I personally like Charleston much better than Savannah but some people like Savannah better. When I go to Charleston I usually stay somewhere on the peninsula or close by. If you want more of a beach scene, there is Folly beach and Isle of Palms (more upscale). Lots of history in Charleston. Ft Sumter, Waterfront Park, The Battery and the colonial homes along E Bay and E Battery St. Patriots Point has the aircraft carrier the USS Yorktown, but may be more of a guy thing. The pitmaster from Franklins Barbecue in Austin opened Lewis barbecue in Charleston. I havent been there but heard its great. Tons of great restaurants around Market St and south along E Bay St. Charleston is known for their food and usually long waits, so if you can do reservations, its recommended. We did a ghost tour that was pretty good as well. We were a bit underwhelmed by the plantation tour that we did, but we did a lower end one, so a higher end may be better. I wouldnt stray far north of Calhoun St as it gets a little seedy.
Savannah:
Havent spent a lot of time in Savannah. Spending time on River St overlooking the Savannah River is normally a good time. Lots of restaurants on River St and Bay St. Hilton Head is a laid back beach community about an hour north and is normally where we go.
Happy to answer any questions if you want to send me a PM.
Bill