I think it would be better to pay cash and try and negotiate a discount in the process, as long as you ensure you have enough funds left on deposit for emergencies. If you subsequently don't like the car or want to change or sell it, having it on an HP deal is an additional complication.
I run a car replacement savings fund, and every month set aside a certain amount so that whenever the current car needs replacing, (and that's not too ofen as I keep my vehicles for years) I have the money available for it. (And on past occasions, paying cash has always got me a better deal).