Both cars are similarly priced, but the Tata Sierra Adventure Plus variant has a few more features than the Harrier Pure X. The Sierra also has a slight edge over the Harrier when it comes to ride comfort, thanks to its Frequency Selective Dampers (FSD). Also, being the newer car, the Sierra feels more desirable, too.
That said, the Tata Harrier has a larger and more powerful 2.0-litre diesel engine, so performance is a lot more effortless in comparison, especially on the highway.
In summary, if you want the newer, better-equipped car, and you’re usage is majorly in the city, the Sierra is the one to go for; but if you drive on the highways more often, go for the Harrier, for its larger diesel engine.