Voluntary Repossession Of a Car?

I have a car that I owe $21000 on but is only worth $8000. I am the only one working and cannot afford the payments. I went to a dealership and I was told that I could do a voluntary repossession of the car, and it would not hurt my credit as much as an involuntary repossession, where they just take it. From what I've seen, it's an either/or proposition. It's still going to hurt any way I go. One source suggested to sell the car and take out a loan for the difference. Or I can just tell the company where the car is and come and get it. What solution, if any, would be better?