The Tata Sierra is a smaller car than the Tata Harrier and Tata Safari, which are in a segment above. But if you like good looks, then the Sierra with its stunning design wins hands down.
Also, the 1.5 diesel is cheaper to maintain than the 2.0 diesels in the Harrier and Safari. So, in view of your preference for a diesel Tata SUV, the Sierra is the best choice.

