If you’re choosing a Volvo XC60 for the Kenyan market, I’d generally choose the diesel—especially if you drive long distances.
🚙 XC60 Diesel
Best for:

Long-distance driving, fuel economy and strong pulling power.
Much better fuel economy than the petrol.
Strong low-end torque, making overtaking and hills easier.
Excellent for highway driving and frequent long trips.
Diesel engines such as the D4 have good torque and are well suited to the XC60’s weight.
Potential downside: diesel emissions systems such as the DPF/EGR can become expensive if the vehicle is mostly driven on short urban trips.
⛽ XC60 Petrol
Best for:

Mostly city driving, smoother/quieter driving and lower annual mileage.
Quieter and smoother than the diesel.
Better suited to frequent short trips.
Petrol engines generally avoid some diesel-specific issues such as DPF regeneration.
Usually higher fuel consumption, especially in Nairobi traffic.
Turbo petrol versions can be very quick and enjoyable to drive.
My choice
For Kenya:
Mostly highway/long-distance → Diesel D4
Mostly Nairobi/short trips → Petrol
High annual mileage → Diesel
Low annual mileage + comfort → Petrol
If you’re looking specifically at a 2019 XC60 D4 versus the 2019 petrol T5/T6, I’d lean toward the D4 for overall running costs, provided the particular diesel has been properly maintained.