You may like to watch following video.
https://www.youtube.com/watch?t=3&v=wM5Th-ComR4
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.