Neither the Surface Pro or Book "excel" at being a tablet, they lack the touch apps, and the Book isn't a great tablet- heavy, poor battery life... It's a better than nothing tablet that you can detach if the keyboard is somehow in the way.
The surface Pro 3 does excel at being a tablet. It has touch apps and even regular apps now can be easily used with touch. The tablet part of the book isn't heavy at all. Again, use a surface pro if you want longer battery life. However, "Pros" use a tablet less than full OS the majority of the time. iPad Pro is an iPad plus with a stick poking out of the bottom when you need more pencil juice. the battery on my surface pro pen is still going after more than a year of use.