Both were published around the same time, and I've used both before for different countries, so I'm happy with their layout and content. However, choosing the right guide this time seemed far more difficult.

Both contained detailed information about the places I plan to visit. However, I found some bits were present in one book, but not the other, and vice-versa. In the end I decided to go for both books. I'm travelling in South India for 3 months, so I thought it would be worth the luxury of having two guides.

Having read both now, I can confirm that both are indeed excellent and each would provide an excellent insight into an amazing country.

Just to prove my point however, here is one example of an item (that I think is critical) that differ in books.

Apart from a small section dedicated to 'Todas', the Rough Guide doesn't really mention trekking in Ooty, which is one of the highlights of the region. Lonely Planet dedicates just under half a page to this essential activity.

Anyway, I don't expect both books to be the same, and here is a summary of the strengths of each book, I hope this might help you decide which book to buy:

Lonely Planet.
Travelling around by train.
Essential information.
Maps.

Rough Guide.
At a glance section on Highlights of South India.
History of India and its regions.
Food and drink.

All in all, I don't think you'd go wrong with either guide and wish you a safe a pleasant journey through this magical country.