I haven't had the opportunity to examine any of the "Best of" guidebooks, but if you check the Specifications you'll see they are only about half as long as the regular guidebooks but weigh about the same (better-quality paper being required because of the color photos). The "Best of" books cover fewer destinations in each country and probably have somewhat less detail on the ones they do cover. You can check out what is said under "Is This the Right Book for me?"
The full color is good for a book you use at home as you decide where you'd like to go, but if you have a good bit of time in a single country, you may prefer a book with broader coverage. And for carrying with you, you may find the weight of a full-color book an annoyance.
Edited to add: If you decide to go for the full-content "Germany" book, be aware that a new edition is due out in December. The "Best of" book will not be replaced for another year, so it certainly wouldn't be a mistake to take advantage of the current sale price if you decide you prefer that one.