Although the process with Cerec and E4D is the same there are differences in the details. Cerec captures the digital impression with a camera and the tooth surface needs to be evenly coated with a special powder. E4D uses a laser system that requires multiple images but does not need powder.
The design software is different but the user experience is similar. Each has special tools to aid in proper design including libraries of dental anatomy. Which is better depends on training and experience.
The milling machines are again different but similar. The differences include size, noise levels, memory, and special features like automatic bur changes. However the essential element is that both deliver high quality results in minutes.
The materials are the same. Both Cerec and E4D use special blocks from Ivoclar and 3M ESPE.