I was going to India on my honeymoon and I bought both the Lonely Planet and Fodor's for the planing.
Came back from India about a month ago and ended taking the Fodor's for the trip, not the Lonely Planet. Why?
LP has a lot of very detailed info -mostly quantitative: prices, time tables, etc- that is very useful for planing your trip and is geared to users that travel without an itinerary. This is very good for travelers that are very price conscious, unstructured travelers, backpackers and doitallyourselfers that make decisions on the fly and hence need all of this very detailed info, missed the train to Agra, should I take a bus? Where? How much? The only bus stops at Shimla, should I stay? What is there to see/do?
On the other hand, Fodor's has info of a more quantitative nature, that is more useful when you have a more structured trip. What restaurants to visit, what hotel to stay at, etc. There is a big emphasis on the epicurean traveler, costs be dammed, you see it the way the book is organized, were LP talks about the eateries by zone, Fodor's talks about types of food, no matter were the place is, it is assumed that visiting the place is more important that going to the other side of town to do so.
Whenever I stuck to the Fodor's Choice or the Fodor's Recommends, I was gladly surprised by the quality of the service/place that was recommended, the best restaurants, best bars, best experiences, best hotels in every town, they are all there, I even surprised some locals.
At the end, you have to decide what type of travel you'll be doing and buy accordingly.