If you can afford the new car, it should last longer since it's new. In addition, the used car may not have gotten the correct oil. There are supposedly some issue with worn camshafts due to the specific requirements of diesel engine oil. Search around and read some more, there is a buyer's checklist for those cars.
Look at the top of the forum and explore the site, there are buyer's checklists for these cars. IMO, maintenance is key to having a long life out of these cars. Because of that, I would go with a new one and start from there. The 1999-2003 ALH engined Golf/Jetta/New Beetles will get the best fuel economy but these cars are getting old. If you can afford it, get a new car. If you are willing to spend money on maintenance and/or do your own work, or are willing to accept that older cars break down more often, go for the older one.
Sounds like you want a 2009, get the newer car if you can afford it. I wouldn't worry about the DPF, emissions have an extended warranty under Federal law and the engine is better. If you know you plan on modding your car, the older car is better. So sounds like the 2009 is best. Good luck. VWvortex has an "ask a salesman" thread in the mkv forum where you can find more info about up to date pricing and inventory.
An older car is better if modding but I still believe that for most people, a new car is less hassles and the new 2009 is a step above the older cars in refinement. Most people don't mod their cars or do their own maintenance.
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