"Is it better to voluntarily surrender a vehicle before it is repossessed or is there no real difference."
*** It doesn't make any difference on your credit 'hit'. It will make a difference on the amount you will have to pay.
Simply, by voluntarily agreeing to the reposession and driving the vehicle to their facility, you will save on the several hundred dollars that they would have pad a 'recovery service' to locate and tow your vehicle.
One other thing to consider.... if your car is sellable, it is MUCH better for you to sell it yourself, even at the payments or below if needed. Otherwise, the lender will sell it at auction, usually for far less than you owe. And then YOU have to pay the deficiency (difference between amount owed and the amount received at action, plus costs).
Example, lets say you owe $5000 on a vehicle, and that you are 'upside down' (owe more than it is worth), with the vehicle worth $4500. Sell it for the $4500 and make up the $500 difference out of pocket and walk away.
Otherwise, that car might sell for $2000 (or less) at auction and you will still owe $3000 ($5000 owed minus $2000 sold) for a car that you no longer have!!!