It really depends on the mileage. General rule of thumb is if the mileage is high use your own car as the mileage expenses will cover all your tax, insuarance, petrol, W&T and them some.
If it's low mileage take the company car.
Or do the maths for 5 mins and work out your monthly mileage and compare that to your BIK.
IMO, cars always cost you money. From tax, insurance, petrol and even when you sell it. However, at least if you sell it and make a loss you'll have the extra cash in your pocket and still have the use of a company car with no monthly payments for petrol,tax, insurance W&T etc.