There are no "Different types of engine capacities", that is just idiot advertisers not knowing what they are selling. The diesel engines available here were 2200cc and the Petrol engines were 2400 cc, both 4 cylinder. That's it, full stop. Other markets had a 3.5 V6 but I don't think any made it here.
Obviously the diesels are more expensive because the Sri Lankan market prefers even their SUVs to be cheap to run so whenever there is a diesel option it will have greater resale value. In the case of the Sorento the diesel is also considerably more powerful and has more torque than the petrol so it will be better to drive. On the downside, the Hyundai/Kia CRDI engine is not as tough as an old school Jap diesel and is more like a euro model in that it needs to have been run on super diesel, needs to have the correct oils etc. Even then, you're likely to be looking at an engine rebuild when the vehicle reaches around 180-200k Kilometers.
Petrols are very thirsty, an AWD model will get around 4-5 KM/L at best in the city and maybe 8-9 outstation. In terms of resale there are LOTS of diesels in the market and far fewer petrols but the diesel should in theory be easier to sell if you get a good one and take care of it properly.