I've been to both. Both are good. Both are quirky.
Key West is more of a tourist destination. Savannah gets tourists but is a small city.
I'd say Key West has a big bar culture. Savannah has that atmosphere on the River Walk but that is only a small part of Savannah.
You have history in Savannah. There is a fair amount of revolutionary war history in Savannah. Sixty percent of soldiers that fought in the Savannah area in the Revolutionary War were of African descent. George Washington had hired Haitian mercenaries. The British had former enslaved African soldiers fighting for them.
You have art. SCAD has a nice museum and several things you can tour.
It will be a good weekend to go to Savannah has it won't be grindingly hot.
We went to a good comedy club in Savannah. I'm not sure if it is still running.
There is a free shuttle across the river. I think we got around town on a free shuttle bus.
We last visited in November and we went to a lot of local activities like local art shows.
Key West--
I'd look into visiting Dry Tortugas National Park
https://www.nps.gov/drto/index.htm
You can get there by boat or by little sea plane for the day. It is fascinating to walk around.
Dr. Samuel Mudd, who set the broken leg of John Wilkes Booth, was imprisoned here.
You also have the Hemingway House in Key West.
I've been to Key West once. Once was enough. Key West is known for its bar culture.
I would return again and again to Savannah but if you go to Savannah look on line for activities for locals. I think those are the most interesting.