Benefit of publishing large articles/papers instead of books
As always, it depends. First of all,


  publishing a book was more prestigious than publishing articles


depends on the area. In parts of the humanities, writing a book is an almost-necessary condition for tenure, but in my area of CS, a book is viewed as best as a waste of time pre-tenure. 

In terms of absolute prestige, books and articles serve different purposes. Even a 200 page article might be a technical exposition of a single result. Witness for example the ongoing series of articles on graph minors in mathematics. A book often tries to distill and put in perspective a body of work, and (sometimes) might have educational components like exercises/problems. 

A well-written book gives you some cachet as an expert in an area, but a seminal journal article can do the same. It really depends on what purpose you expect the document to serve.

A final point. A book (even a technical one) may not be peer-reviewed in the same way as an article. Indeed, it's not common in my area to have brand new research appear first in a book (I'm excluding simple observations and recasting of results).