A lot depends on what car you would want and how good you are with cars, garages and so on. Personally I would take the £350 per month, spend £500 or so on a 8 - 9 year old car that isn't worn out and use that. It's not clear from what you say how the fuel and private/business miles will be dealt with but, if you receive 40p per business mile and it costs you 15p, you keep the difference with no tax payable (for the first 10000 business miles per year).
I know many other people prefer the piece of mind of having the company responsible for owning and looking after the car.