Company Car or Allowance?

Okay, this is my first post so bare with me..

I am due to start a new job in Feb 2013, the basic salary is £38,500 with a £5,700 car allowance, or company car, which I can select from a list.

My question is which will I be better off with? I am getting really confused with the higher rate tax on company cars - If I take the car I will still be at the higher tax rate so does this mean my monthly tax on the car doubles?

for example an audi a4 i have my eye in £82 for 20% tax payers and £164 for 40% tax payers. Do i automatically have to pay double even though i am only just in the higher bracket (38,500)?

or should i just take the full salary of £44,200 (incl car allowance) and buy/run my own car? would this be better from a tax perspective?

I am finding all this very confusing to work out exactly what my take home pay will be. Can anyone help??

Thanks