therubyracer is unstable between platforms, platform versions, ruby interpreters and even ruby versons. I personally couldn't install it on non-ree 1.8.7 (legacy project) and Windows (railsgirls), and had issues installing it on other combinations. Pretty much everytime there's native extension compilation issue, it is therubyracer. On the other hand I never had issues installing Node.js.
therubyracer is officially discouraged by heroku as it uses lot of memory. On the other hand node.js advertises itself as very lightweight.
using therubyracer on server is just like installing node that can be used only by ruby apps (you're using v8 anyway)
btw. node.js is necessary only on Windows and Ubuntu development, on Mac development ExecJS is using JavaScriptCore.
Reasoning
btw. node.js is necessary only on Windows and Ubuntu development, on Mac development ExecJS is using JavaScriptCore.