I have only selectively read RS Sharma ( I am assuming you are talking about his “Indian Feudalism”)
Both Romila Thapar and Upinder Singh I have read completely. From that, I can tell you the following.
- From an examination point of view, Upinder Singh is the much better book. It simply covers a lot of ground primarily because the author herself mentions in the preface that she wrote the book to cover the critical gap due to the lack of a good text book at the undergraduate level.
- Upinder Singh succinctly sums up and covers most of the historical debates regarding various aspects of Ancient and Early Medieval Indian History.
- Romila Thapar’s Early India was written as an introductory book for Indian History. The author herself mentions that an undergraduate student would be better off referring other books. However, I would recommend this book to anyone who wants to study history of ancient India.
- Romila Thapar, even though she does cover South Indian history in her book, doesn’t delve into much detail. Given the direction UPSC History papers have taken in recent years, it would be too costly to ignore these sections. Upinder Singh covers these parts very well.
- If you are well versed in English, Romila Thapar is a delight to read.
- Upinder Singh is a capsule which contains a lot of curated content. If you read her book, you can skip reading Romila Thapar, Neelkanta Shastri, RS Sharma etc. You get the gist.
From an exam point of view, if you have to chose one book, I would say you go with Upinder Singh. If you have time, read both Thapar and Singh. If you absolutely love the subject and have time, read all three.
All the Best