Expanding Ben's answer, why do not you take a look at how StackOverflow handle this?
Take your own thread for example: https://stackoverflow.com/questions/4479453/adding-short-urls-to-a-blog-makes-seo-better-or-worse
is resemble "nice friendly-URLs" of your blog, while https://stackoverflow.com/questions/4479453/
is the shorter one, similar to your www.blog.com/:157
Now if you open this source of this page, you should see rel="canonical"
that Ben elaborated above. This tag makes sure that search engines will index the assigned one. Moreover, it is advised that those two links should be clickable, so that users can bookmark the canonical one. Do not miss this thread: Why do some websites add "Slugs" to the end of URLs?, it is not tackle your question, yet it gives your deeper insight on your understanding.