The 1.9 PdTdi is a newer engine and is less polluting and economical. On the other hand, the 2.5 Tdi has 6 cylinders, 24 valves and it puls from tick over ... If its sheer power you after, then get a 2.5 tdi quattro with the 6 speed manual gearbox and get it chipped to about 200 Bhp and it will still return about 40 Mpg !