I’m looking to buy a new compact SUV and am conflicted between the Sonet 1.2-litre petrol and the Maruti Brezza (1.5-litre petrol). My main concern is fuel efficiency - I’ve heard the Brezza’s 1.5 is more efficient than the Sonet’s 1.2. I’m also considering the Sonet diesel manual. My monthly running is about 1,000-1,200 km, mostly on highways. Which one should I buy?

Given that your usage is relatively high and mainly on the highway, you should go for the Kia Sonet diesel MT. Its strong pulling power compared to the naturally aspirated petrol engines you have mentioned will allow for more effortless cruising and overtaking, and the fuel efficiency will be considerably better too. 

You should also consider the new Hyundai Venue, as it is a bit more modern and more spacious than the Sonet. As for the fuel economy of the petrol engines, it depends on your driving style and conditions. Since your driving is mainly on the highway, the larger, more powerful 1.5-litre petrol in the Maruti Suzuki Brezza would likely be more efficient on the whole, as it would be less strained at higher speeds than the Sonet's 1.2 petrol.