I recently made a road trip across the American Southwest and purchased both the Lonely Planet 4th Edition (December 2005) and the Rough Guide 4th Edition (October 2006) as my guides for the week-long adventure. Lonely Planet emerges as the clear winner when the two are compared side by side. It is less than half the thickness of the Rough Guide book, yet has more coverage per town or natural site. I commend Lonely Planet for using such a thin but readable font and thin, high-quality paper to make this guidebook have such a small footprint.
The Lonely Planet guide is divided into tabbed chapters by state (the guide covers Arizona, New Mexico, Utah, Las Vegas, and Southwestern Colorado). Maps cover both regions and downtown areas of the major cities and towns in the Southwest. For each city, restaurants, hotels, attractions, and essentials like internet access, post offices, bookstores, and medical facilities are listed. The sidebars contain information from a historical or side note perspective--these are perfect for reading aloud during the inevitable driving time for any Southwest adventure.
If you are planning to spend a long time in the Grand Canyon or other attraction area, I recommend purchasing an additional supplemental guidebook. As a road warrior, I appreciated that this book has all the essentials, but leaves week-long trips to Las Vegas or the Grand Canyon to other guidebooks.