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Not done a slackbot (yet?), only a few small app and a large 7-yr-project, but with Rails it's really about whether or not you love Ruby. There's still Sinatra but that's about it in regards to web-frameworks. Rails is perfect for punching out a prototype quick and heroku (which started as an online Rails-editor) make delivery very easy.

Try ruby and see if you like the language.

Hanami is growing in popularity and Roda is a rock-solid and faster alternative to Sinatra.

Both worth considering.

I used Rails because that's what I am familiar with. I don't tech stack makes any difference though.

Well leveraging rails and quality plugins makes for extremely short development time. Author mentions low server load for this app so prime candidate for rails imho.