Whether you choose to go to a franchised main dealer or an independent car dealer, they all use the same industry price guides to work out the value of your car.
There may be fractional differences in the prices quoted in these guides for the same car, but the bigger differences in valuation will be due to how generous the individual dealer.
This will be dictated by how easily the dealer thinks they can shift your car. The easier your car is to sell on and the less the dealer will have to spend preparing it for sale, the more likely they are to give you a high valuation.
However, remember that all dealers will err on the low side of a valuation as this will improve their profit margin when they re-sell your old car.