Of the three options you mention, I have only used RefineryCMS on Heroku (for 3 different websites)
As you have lots of Rails experience, I can recommend Refinery. If you didn't know rails I wouldn't say that as there is a wee bit of a learning curve getting up to speed with Refinery. For straight vanilla CMS stuff, then Refinery is very easy to get going with, but you can also customise it yourself without huge hassles (or by spending cash up front) later on. There is plenty of other info on the internet re. heroku vs aws. I think the general consensus is that Heroku is easier to get started with, but (as always) it depends! I know someone serving 250,000 page hits a month from a free Heroku account with rails.
I expect this question will be shut-down as it is more of an opinion-based question rather than give me the facts... Anyway, hope this is somewhat helpful to you.