Choosing between WAS Classic and WAS Liberty

“There are a number of aspects to consider when choosing between the traditional WebSphere Application Server (WAS full profile, or WAS Classic) and WAS Liberty for deployment. If you
have a hard requirement which is only available on one or the other (such as a particular API) then the choice is easy, but as the Liberty function has grown, it has become more common to
need to weigh the pros and cons of each more closely; to consider topology choices, operational capabilities and security options. Over one hundred and fifty IBM products have shipped with
Liberty as their internal application server and an ever-growing number of IBM SaaS offerings, such as BPM Workflow and Watson Analytics, are running on Liberty.”
See this document from IBM