I dont much like Cadillac in general, so I would lean toward a Merc or Lexus, regardless, but the Caddy should be cheaper to repair if anything goes wrong. I am not familiar with the SUV market, so I dont know how seasonal it is. I believe there is a spike for non-sport cars/trucks during Xmas then a taper in Jan - March. If you wanted optimal monies, you could take a chance, buy the Merk in January and hold the Civic till May-ish to possibly get a few more grand in private party sale. Alternatively, sell the civic now, and make her take the bus (like a Alpha).
Regardless of what the dealer actually has involved in the car, if you can get with in 10% of auction price, you are getting the car at fair market value. The GLK for 23.5k seems like a fair price from a dealer, as long as everything checks out.
The Civics that are crossing the auction block at a little crazy with the high mileage...
They are a popular car, and I would think, a low mileage civic @ $8,000 would sell pretty fast if you had to go private party.