The choice between a diesel and a petrol really comes down to how much you drive and how long you intend to keep the car. If you drive a lot and will put many miles on your car then a diesel makes sense as only then would you recover the extra initial investment. Between the Kia Seltos and the Tata Sierra it really comes down to your specific requirements which we don’t have.
However if you are going for a diesel, the Kia would be better as the diesel engine is more refined and delivers a strong performance too. If it is petrol you are looking at then you can consider the Sierra but the turbo direct injection engine only. The naturally aspirated unit is under powered and lacks a lot in performance.