Ibiza: 66kW vs. 77kW PD - Differences Explained

Hello.
I'm planning to buy a new Seat Ibiza. The 1.9 TDI engine comes in two power variants. Can anyone tell me what the exact differences are between them? Is it just a software difference, or are there also hardware differences?
Can anyone tell me if, for example, the 77 kW engine has a longer gear ratio?
The Ibiza is supposed to replace a Golf III with an AHU engine, which has reached 300,000 km. This raises the question of whether the "older" common rail (PD) engines with diesel particulate filters (DPF) are actually mature technology now.
are??
Thank you for your efforts.