When comparing Assemble vs Octopress, the Slant community recommends Octopress for most people. In the question“What are the best static site generators?” Octopress is ranked 4th while Assemble is ranked 15th. The most important reason people chose Octopress is:
Octopress is designed to remove all the time-consuming and frustrating tasks you usually have to deal with when using Jekyll. You don't have to write your own HTML templates or do any configuration to get started. The default template also takes care of any basic CSS/JS you need to write to get going.
Specs
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Pros
Pro No dependencies on Ruby, Python... just JavaScript
Pro Built on node.js
Pro Powered by a popular template engine Handlebars
Handlebars is the default template engine for Assemble, but you can add any template any you want.
Pro Highly customizable
Pro Markdown support
Pro Highly extensible
Assemble can be extended with plugins/middleware, helpers and mixins.
Pro Nested layout support
Assemble makes it easy to work with layouts. Layouts are used to "wrap" pages with common page elements, such as a header, footer etc. You can even nest layouts!
Pro Use mainstream build tools Grunt or Gulp
Pro Very easy to use
Octopress is designed to remove all the time-consuming and frustrating tasks you usually have to deal with when using Jekyll. You don't have to write your own HTML templates or do any configuration to get started. The default template also takes care of any basic CSS/JS you need to write to get going.
Pro Lots of plugins (from Jekyll + 3rd party)
Another advantage of being based on Jekyll is default access to the extensive plugin system for Jekyll. There are also plugins developed specifically for Octopress.
Pro Based on Jekyll
As Octopress is simply a framework on top of Jekyll, it benefits from being based on one of the most popular site generators available. The primary benefit is that it is backed by git and is built in such a way that the site can be easily re-generated if the content changes.
Pro Uses Markdown for writing
Uses the Markdown syntax for writing blogs.
Cons
Con Documentation can be hard to navigate
Especially for someone new to Assemble, it can be difficult to find what you're looking for in the documentation.
Con Not Actively Developed
Last commit is Feb 22, 2016, Social media hasn't been updated. Dead project?
Con Inefficient
Adding a new post, for example, causes the entire blog to be regenerated.