The best place to get more info on this is the study guide for each of these exams .
The course recommended for App Builder is the same course that was needed for developer exam before .Remember that 401 is not the exam name and it was a course name .
Both the exams focus on the declarative features of the platform .The best place to get what topics to be covered is documented in the Study Guide
The difference I found between developer exam and App builder is App builder also focus on the latest features added to the platform like Lightning App Builder , Salesforce 1 Mobile Configurations ,Process Builder Automation,App Deployment and Application Lifecycle management.So Apart from reading previous content for Developer exam also focus on these Topics .
Platform Developer I
Platform 1 is very different than previous developer exam .This exam now focus on both declarative and the programmatic capabilities .It does not test your in depth knowledge of Apex and Visualforce but basics of apex and visualforce is tested .You should have clear knowledge of when to go for declarative features and when declarative features wont full fill your need .Again Study guide is your best friend here .You should have great command over building data models and logic using declarative and programmatic features .
Platform Developer 2
This is similar to the advanced developer exam we had previously .It includes multiple choice followed by assignment and essay exams .The candidate is also expected to know Integrations and best practices for app development and deployment .