I must admit, I'm usually more of a Rough Guide fan than Lonely Planet, though alongside how the book has been set out and its format, it's more often than not the author of the guide that makes me pick which one.
However, this proved to be a different case.
If you are going to spend a month or more in The Philippines, there's little to choose between the Rough Guide and Lonely Planet - both are good all-round books, being informative and well written. Yet on this trip, I only had five days of travel - two spent in North Luzon at the Rice Terraces, and three in El Nido in the Palawan Bacuit Archipelago. For such a short journey, buying Lonely Planet's individual chapters is the better option - I only bought the two chapters I needed, rather than the full book, and these took up a tiny ammount of space in my backpack.
My general conclusion - if you're going for a long time, buy the Rough Guide; if you're going to limited places on a very tight schedule, buy Lonely Planet's individual chapters (these are also downloadable and can be printed out when you need them, so you don't have to carry them halfway accross the world with you!)