I prefer linode, aws or so.
linode : is a standard linux server. you can login, install 3rd party dependencies and play around just like in your own server. Installing nginx/rails/ruby is the same way as what you did on your own pc/laptop/server.
heroku : is a service. I have to learn lots of stuffs that are not valuable at all if one day you switch to another platform(e.g. linode) or you have your own real server, for example, check the logs, install databases, or install gems. I have printed out most of its documents and read them in 1/2 days, and then I realize that I can't use these knowledges in my working server( that my company offered to me)
linode is cheap enough, $20 per month.
- heroku is not always free. and I don't think it's stable enough for demonstration purpose.(e.g. your free heroku app will fall in sleep in spare time, and will cost you several seconds to wake up. this SEVERAL seconds is long enough to make you lost your customers if they want FAST web app )
so, forget heroku, buy or setup your own VPS, use it for years, then you will be an linux expert.