According to the website it looks like platform.sh has a focus on drupal, symphony, & php while runnable is platform agnostic.
Anyway, this is true we currently focus on the PHP market. Though you can already deploy nodejs applications (or a mix as we support complex multi-app clusters).
The platform itself is totally abstract so we will probably add in the near future many other run-times.
The main difference I think it would be interesting to point out is that Platform.sh is a production system... not a staging thing.
What we clone into staging is production .. with all the data.. however complex it is. And we do it really fast.
And we run production in a multi-datacenter / scalable / highly-available setup.
BTW we are hiring like crazy so if you want to work on your pet language having our features... contact me :) We are a fully distributed company... so u can be anywhere the world.
You don't have to run production on Runnable. We are complimentary to your existing prod system. Most of our customers run on AWS.