When comparing Blocks.js vs Preact, the Slant community recommends Blocks.js for most people. In the question“What are the best JavaScript libraries for building a UI?” Blocks.js is ranked 10th while Preact is ranked 16th. The most important reason people chose Blocks.js is:
Blocks.js is just 16.5KB minified and gzipped. So it's pretty small.
Specs
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Pros
Pro Small footprint
Blocks.js is just 16.5KB minified and gzipped. So it's pretty small.
Pro Modular structure
Apps developed with blocks.js are modular in structure and make use of reusable objects. This makes these applications very maintainable.
Pro Lightweight
The size when gzipped is only 3kb, and performance is as good as (or even faster than) React itself.
Pro Same API as react
Avoids having to learn an entire new paradigm, and makes it easy to switch between the two libraries later on.
Pro Lots of examples and libraries
They have a bunch of examples and addon libraries.
Pro Awesome CLI
Preact CLI gives you a perfect scoring PWA out-of-the-box with no configuration.
Cons
Con Relatively new
Blocks.js is a relatively new library. It doesn't have much support and there re not many people who use it. So if you run into any problems it would be hard to get solutions.
Con Difficult to learn
You need to be good in React JS first to learn Preact. Documentation is not rich. And some of the examples given on site are not working. Developers are not much active on Stack Overflow as well.