Book Vs Blogs

I know this is going to be a biased group of readers, but what do you think is better: Writing a book or Writing a Blog?

My friends these days are trying to convince me to write a book. They say that a book a more structured and the author actually thinks before writing a line in a book. As compared to that, the blogs are random rambling of a person who himself might have never even proof-read his posts.
They also say that a book is very difficult to write, because the person actually has to take time out everyday for a month/year to write, while a blog is something that is written while killing some excess time between flights/busses or even while waiting for the coffee to be served in a coffee shop.
A good author chooses quality over quantity. While writing a book, the author needs to handpick from his thoughts and observations and write only about them. While a blogger uses a shotgun approach. He/she would write anything and everything under the sun, to maintain their ratings they need to saturate the minds of the authors with daily/periodic updates. No wonder, like a newspaper, once read their posts are deleted/forgotten for ever. While a good book is always stashed. A whole library often grows around them.
Bloggers can be find a dime a dozen. Everybody and anybody, irrespective of whether they want to say anything or not. While a good book is a real treat to the eyes and the senses of the reader. Their words alone are so powerful that they do not need to use aids like colorful fonts, and pictures to prop their deficiencies in writing.

Although most of the points are valid and strong, in my defense I could only mutter:
Blogs is all about brevity. A serious blogger knows that nobody has the patience to go through 300 pages of blabbering and repeating of the same point. Hence the blogger condenses the essence of that 300 page book in catchy title and 100 words of post.
A blog is a two way traffic, while the author of a book rarely gets a feedback and never gets a chance to elaborate/correct his stance. If I write a post titled “Books are obsolete”, within 24 hours I would get several comments/feedbacks from the fellow readers both ridiculing and elaborating the idea. However even after months of writing a nice book about “Blogging is for Losers” the author won’t be begging for any response from the audience. You can be assured that half of his friend (even after being gifted a free copy of the book) won’t bother to open the cover page.
Most ideas are not worth writing a whole book on and not everyone is fortunate enough to lead such a colorful life that he/she can collect 50-60 such ideas and then spend tens of thousands of bucks to get it printed. Hence blogging is the most cost efficient and productive way to express oneself.

What do you think is better spending time writing a book or spending time updating one’s blog?