Company car also == More tax.
You pay based on the CO2 emissions and a percentage of the car's value.
http://cccfcalculator.hmrc.gov.uk/CCF0.aspx
Using a new 136HP standard A4 Diesel @ 25k (120g CO2) gives you a tax liability of about £1300/year (assuming no fuel card) which means your tax allowance goes down by 3250.
If you go with the SE Quattro 170PS @ 29k (145g CO2), takes that tax code down by a whopping 6 grand, meaning you would pay out nearly 2.5k extra (assuming you'd be on 40% tax).
I'd go with the allowance option.. If you get a decent accountant, they should be able to get you more money back at the end of the year too.