When comparing Sinatra vs Phoenix, the Slant community recommends Phoenix for most people. In the question“What are the best web frameworks to create a web REST API?” Phoenix is ranked 8th while Sinatra is ranked 18th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Small loading time
Since it has very few dependencies, the loading time for a Sinatra app is considerably small.
Pro Has only the bare minimum needed
Sinatra has taken an approach of having only the most useful components needed to build applications out of the box. It has simple routes along with a Domain Specific Language over a Rack layer.
Pro Very positive reviews
Pro Relatively big community
It has the biggest community from all Elixir web frameworks so there are examples and tutorials all over the place and you can quickly find help.
Pro Easy to learn
The official guides are really well written.
Pro Will be easy to learn for Rails developers
Cons
Con Hard to scale well
Because it's rather small and minimalistic, scaling up is not very easy with Sinatra. You need a great deal of knowledge on libraries and modules that may be useful for your particular use-case. As your application grows larger it may be hard to keep things clean and minimalistic, losing a lot of the advantages that Sinatra has.