I've been trying for weeks to get a Mono environment that will host an MVC4 application (minus async/await). Any suggestions?
You can even experiment with the new OWIN/Katana stuff if you like: http://friism.com/running-owin-katana-apps-on-heroku
What surprises me more as an appharbor customer though, is that you've moved to Heroku! I must have missed that news...
Personally I run everything on a VPS, because I'm not worried about scale. If I was running something larger I'd probably go with Azure.