The newer ones only really pay off if you do a LOT of miles per year. A friend has a sedan and he puts almost 20k per year for work and gets reimbursed. He's way ahead on reimbursement.
We own an 03 wagon and love it, but don't think I'd spring for a third TDI at present usage. We don't drive nearly as much as we used to (not even 10k the past 12 months) so the maintenance wouldn't pay for the engine upcharge.
Bottom line: if you drive a LOT of miles per year, it'll probably save you money over the long run.
Illegitemi non-corundum est.