I did some looking around real quick, and Mk6 is going to be a 2010-2011 with around or over 100k at 10k$ price point, Mk5's, if you find one, you could probably get an 80k one for that price, but Mk5's are kind of hard to find for sale these days with less than 200k, and most of those are 5-8k.

Mk6's are worse MPG when compared to Mk5's on paper, and I heard that they get 50mpg with ease compared to Mk6 that requires some hypermiling to reliably cross that mark. Main improvements from Mk5 to Mk6 is better aerodynamics to improve cabin noise, newer interior, 6 speed manual or DSG instead of 5-speed or regular automatic and an engine with a good bit more umph.

Keep in mind that newest Mk5 TDI you can get in U.S. is an '06 afaik, and that ultimately Mk5 and Mk6 are not THAT far apart, Mk6 is often characterized as Mk5.5 since the chassis hasn't really changed. Main improvements from Mk5 to Mk6 is better aerodynamics to improve cabin noise, 6 speed manual or DSG instead of 5-speed or regular automatic and an engine with a good bit more umph.

I feel like I'm rambling, but overall I'd say at your price point I would just get a Mk6, if you were sub 8k, I'd suggest a Mk5 with manual.