If the car used for business is newer than 5 years, there is no income tax on the car allowance.
Are you in the US? If so, then there is no tax rule that a car allowance for a car newer than 5 years old is not subject to income tax. I would be interested in hearing the basis for that assertion.
I agree with the others that, tax advantages or not, it still makes a ton more sense to keep driving the Jetta.
I think that what has happened here is that the company has determined that a car newer than 5 years, combined with the usual business use of the vehicle clearly justifies the car allowance - on a car older than 5 years, the allowance could be construed as too much, (indeed, this is what the OP is trying to do here).
http://www.irs.gov/publications/p463/ch06.htmlBack to the question, to decide whether it is worth it to buy a newer car, you'd have to look at your marginal tax rate to determine the tax savings, then decide if that is enough to cover the additional cost of the taxes - if you are in the 25% bracket, you save $109.25 per month from the fixed amount on taxes if you get a newer car, plus $.0525 per mile for the variable part. If your husband's business use of the vehicle is 1000 miles per month on average, that's another $52.50. So in that scenario, you're getting an extra $161.75 per month from the tax savings by having a newer car. Is that worth it? Depends on the cost of the newer car.
My gut feeling is that you are probably better off buying older vehicles, and driving them into the ground and just paying the taxes - you could deduct your actual business mileage in this case (if you itemize), which might work out reasonably well on the taxes any way. If you decide to go with the newer car route, either for financial reasons, or maybe to impress clients or whatever, you're probably better off getting something at least 1 or 2 years old than something brand new - cars depreciate very fast early on, particularly if you are putting a ton of miles on them as your husband might be doing here. Then you'd probably have to buy a new car ever 4 or 3 years to stay under the 5 year cap, so this comes with that hassle as well.