You may like to watch following video.
AppBuilder Transition Certification focus on declarative skills. It do not focus on Visualforce at all now. It do focus on Lightning Introduction, Salesforce1 understanding, Sandbox and Packages.
Platform Developer 1 focus on coding skills. It also focus on Introduction to Lightning.
There is no prerequisite for Platform Developer 1 certification now unlike DEV 501 for which we had to be certified developer was prerequisite.
Platform Developer 2 focus on advanced coding skills and also has assignment.It also focus on Integration skills(SOAP API, REST API, Analytics API) now which were not tested earlier in Advanced Developer certification.