Your observations are correct. Neither the Tata Sierra nor the Kia Seltos has been crash tested by Global NCAP or Bharat NCAP yet. But Tata Motors cars have had a good track record with these tests. While the Kia Seltos uses an all-new global platform that should be safer than the outgoing model.
Both SUVs feature the latest in active and passive safety features, which should be a reassurance, and thus, on the safety front, you won’t be wrong to go with either. As for other factors, the Sierra is certainly the more spacious of the two and edges the Seltos out on comfort as well.
However, in most other areas, it’s the Seltos that comes out on top, including, as you have pointed out, after-sales experience. The Sierra is being produced at the ex-Ford plant in Sanand, Gujarat. It is said to have higher manufacturing standards, and the brand has been committed to improving its after-sales service across the country, but the results of that will take some time to show.