the 170 TDI found in mk5 is PD TDI.
The new 170 in the mk6 (GTD) uses
TDI® with Common Rail and Piezo Injectors The latest generation common rail system has one high-pressure pump and one rail per cylinder bank. The maximum injection pressure is 1,600 bar, 250 bar more than previous common rail systems. When piezo injectors are used for injection, a voltage is applied to ceramic, which changes its crystal structures. The result is a minimal geometric change, which mechanically triggers the opening of the injector needle. The use of piezo injectors makes it possible to reduce the moving mass at the injector needle from 16 to 4 grams in each case. This enables considerably smaller and more precisely metered injection quantities. At the same time, piezo technology allows a higher injector needle speed.
The number of injection processes per combustion cycles can thus be varied almost at will. The 3.0 V6 TDI engine, for example, has up to five injection processes: in addition to the main injection process, double pilot injection takes place at the lower end of the speed range and single pilot injection in the medium speed range. On top of this, single post-injection takes place up to around 2,500 rpm and at maximum part-load. This reduces emissions and ensures a smoother combustion process.
Heres even more info on the difference between PD and common rail.
http://www.automobile.com/vw-to-switch-to-common-rail-diesel-technology-from-pd-tdi.html 
I've had both and driven over 80000 in each and can confirm the new 170 is wicked.
