I have the same situation, and the short answer is that I recommend using one linode and using virtual hosting at the web server level to handle traffic.
I have 6 websites running on my Linode right now (and over the last 5 years), 4 Ruby / Rack applications, one PHP application and one simple static HTML application. Since in racks each one starts a different instance of the application, if someone does not work, the rest remain alive.
I'm more concerned about memory / replacement due to exceeding the expected amount of traffic or memory leaks. I recommend installing monit and setting it up to restart processes if memory problems occur.
source share