Taking the company car will also be a taxable benefit.
http://www.hmrc.gov.uk/calcs/cars.htm
Using the calc shows you'll be taxed on an amount of £3960. This being £792 per year if she on the 25% tax or £1584 if she's on 40%.
Taking the £300 per month (ie, £3600 per year) will be taxed too at whatever your rate is. So this would mean your yearly salary after tax is only increased by £2700 (if on 25% tax) or £2160 (if on 40% tax).
Therefore taking the BMW means you'll be £792 + £2700 = £3,492 per year worse off (if on 25% tax) or £1584 + £2160 = £3744 per year worse off if on 40% tax.
If you think you can run a car (buy, lease, maintain, tyres, etc) for less than these figures (ie, approx £3,500 each year) then you're better off getting your own. Of course the BMW is peace of mind in that you'll not have to worry about high risk events like the engine going bang.
WARNING - Its been a long day, so I don't guarantee my maths. But someone will be along to check it I'm sure
