I'd go for the 170tdi, you'll say i'm biased cause I drive a diesel. But the tdi can be remapped easily to 200 and higher with the dpf removed. 170tsi hasn't shown great gains with maps from what i've read, reliability issues have arison. I have driven my cousins 170tsi and was impressed with the power from such a small engine.
But If you can stretch to a gti then do, different breed of animal. Otherwise as I said i'd opt for the oil burner. sure, off the line, the petrol is likely to hand it's arse to it but when they get going those 170tdi's going, they are no slouches,especially a remapped one,great torque. It's all about the torque with us diesel heads ha ha.