There are various websites that will explain the ins and outs. It will depend on the value of the car allowance, how much they’ll pay you per mile, your tax bracket, how many miles you’ll do.
Without knowing any of the above, I would guess taking the car allowance and keeping the current car would leave you best off financially.
When I got a car allowance from work I used it as a way of ‘justifying’ a brand new car on lease.
Don’t forget if you get a car allowance and get paid for mileage, you should be able to get some tax back at the end of the year.