There are *common* things for which you will use your guidebook.  Base your guidebook choice on how easy it is to do these things, because you will be doing them every day:

• You pass a gravel road or a trail crossing and want to know where you are.  Maybe it’s an indication that you’ve gone past a tent site or spring that you were looking for.  The A.T. Guide has about 800 more landmarks like this.  With it, you’ll be much more likely to find out where you are.
• You’d like go into town, for example Falls Village, to get a maildrop.  Falls Village is close, but the path is not straightforward.  The A.T. Guide has a map of the town, so it’s a simple matter.  Not going into Falls Village?  There’s 30 other town and area maps that the A.T. Guide has that the Companion does not.
• You want to know what kind of terrain is ahead.  The A.T. Guide has an elevation profile.  At a glance, you can see the ups and downs.
• Another benefit of the landmarks being aligned with the profile is that the text is spaced proportional to the mileage.  Each data page in the book covers the same number of miles (20.6), so even without reading mileage numbers you can make a good guess at the mileage between landmarks.
• You are in town and want to know what services there are.  Scan the text of each book and see how quickly you can find something, an outfitter for example.  The Companion will provide you with a block of text into which all services are lumped.  To find the outfitter, you’ll need to scan the paragraph (sometimes a page long) to find the italicized word “Outfitters” and then read on till you find the one you want.  In The A.T. Guide, services such as this will all be on a line of their own, preceded by an outfitter icon.  The business name is in bold. You will spot the information you need much more quickly.    
• If you are a southbound hiker using the Companion, you’d progress through the book’s mileage section backwards, from the back of the book to front, and from the bottom of the page to the top.  When you want town info, you’d reverse yourself and move back “forward” through pages. Sounds doable, but give it a try; it’s not easy to break the lifetime habit of reading a book front to back.  The A.T. Guide has a southbound version.  All  information would be front to back, top to bottom.
• You want to hike with just the pages you need.  The A.T. Guide is sold bound or loose-leaf, and every book sold from the website comes with a heavy-duty zip-lock bag.
• You start your day at a shelter and want to hike 15-20 miles.  Will you finish near another shelter?  The Companion gives the distance to the next shelter, but on average the shelter-to-shelter distance is less than 10 miles.  You’d have to look up the shelter after that and subtract their mileages.  If it’s still less than your desired mileage, you’d repeat that process for the next further shelter.  The A.T. Guide gives shelter-to-shelter mileages for the next three shelters.  
• While on the trail and looking at the mileage data, you see a road crossing with a town nearby.  You want to find out if the town has something you need, so you flip to the “Town info” part of the book.  In the Companion, the town info may be up to 12 pages away.  In The A.T. Guide, most town services are a page turn away.  Even so, every data entry with additional “town” info provides a page number to make is easier to find that info.  The “town” entries have corresponding mileage, so you can easily cross-reference back to the data page.

Even with the extra features, The A.T. Guide is ounces lighter than the Companion.

Sometimes the case is made that you can get by with the Companion (which is obviously true), or that any one of these look-ups take just a little more effort using the Companion.  The A.T. Guide makes it easier to do the look-ups that you’ll do hundreds of times. The difference is significant.