Does a smaller application footprint mean cheaper PaaS costs? which language?
Memory footprint will certainly reflect on your PaaS expenses.
But to tell you what to use is hard without knowing more about the project.
Node.js per se is great, but it's not perfect for every case.
Python is very friendly for development, and has ok memory usage, but again - it all depends on what you're doing.
So, I have built and deployed a Grails app onto cloudfoundry.
And as I play around with examining instances & memory I start to wonder;
If my app's footprint is larger because of the technology I chose to develop it in, will it start costing me money sooner rather than later? Surely it must?
If that is the case, am I better off developing in an alternative language? if so which has the smaller footprint (python, ruby, node.js)?
Of course, costs should not determine which language I use, I should select language/framework on merits and personal preference. 
But it is still a question I'd would really like to know the answer to.