I've stayed at both locations multiple times and generally prefer the Penn's Landing location because of the Delaware River/Ben Franklin Bridge views and overall ease of getting in and out of there to/from the airport.
Both locations are close to Independence Mall, the subway and restaurants. Midtown location is closer to Reading Terminal Market, City Hall, Walnut/Chesnut St shopping. Penn's Landing is closer to Elfreth's Alley, Old City, and 2nd St bars/restaurants. Penn's Landing location does provide a local shuttle, too.
So many great restaurants near both locations, depending on what you're interested in.
I haven't tried either Kimpton yet since the merge, but am planning on it.