I like the AAA Guidebooks because they are generally small and can fit in your pocket, something I really appreciate when I am doing a lot of walking.  The downside is they don't take you to out-of-the-way places, for that I like Lonely Planet.  I also like the slightly irreverent tone of the Lonely Planet books which is not for everyone.  I often purchase the AAA Guidebooks to take with me but check other guidebooks out of the library to help me plan my trip.  My local library has a good selection of guidebooks, you could always visit yours and see which book suits your style before you buy one.