I vote that you roll with Heroku until you feel so comfortable with the Passenger/Nginx/Capistrano that you're frustrated with the amount you're spend at Heroku

I vote that you roll with Heroku until you feel so comfortable with the Passenger/Nginx/Capistrano that you're frustrated with the amount you're spend at Heroku because you feel like you could do it yourself. Point is you have enough to worry about, don't get a VPS till you HAVE to.