Why does ionic projects use Circle CI ?
What makes it distinctive than others sayTravis?
If i am building some small app using a starter template should i prefer Circle CI ?
Generally what is the preffered CI for ionic apps?
i am using travis ci because it is easy to use, a german company! it plays well together with other services like coverall and it is free for all public repositories.
Circle ci has also a free-plan.
so it is very subjective… but i am from germany and i like the service… so i try to support local services, if i think they are worth it 
If you want to run a private/own CI-Server you can use gitlab CI. It is free if you are hosting it on your own 
Greets.