It's unfortunate that Scratch is built upon the Flash platform. Though Scratch has been released under the GPL, it requires nonfree software in order to run. Scratch looks interesting, but I cannot recommend its use until this problem is fixed.
"Blockly is a component that may be useful for a variety of projects, including educational tools. We want developers to be able to play with Blockly, give feedback, and think of novel uses for it. All the code is free and open source."
https://code.google.com/p/blockly/source/browse/trunk/COPYIN...
Scratch is free, committed to open source, and is frankly the best software out there for getting young kids excited about coding.
I'd highly recommend it, despite being encumbered by flash.
I have not compared the two, but if you want a freer version, I would recommend looking at http://www.phratch.com/