Here are a very short summary of the differences:
Online ECA:
You have to read all the stuff by your own. You don´t have any Labs. So it's all “theoretical”
Classroom ECA:
You have a Trainer who is teaching the material. Therefore you can ask questions about the slide and the Nutanix topic general. You have to do Lab tasks. You can exchange ideas with other participants.
Online plus ECA:
You have to read all the stuff by your own. You have to do Lab tasks. For the Lab a Trainer can support you.
You will automatically get more information from a classroom training because of the Trainer and other students. However, the content of the material are same.
It is much easier to pass the NCP-MCI exam with practice experience.
Hope this Information is helpfully.
The course datasheet https://www.nutanix.com/viewer?type=pdf&path=/content/dam/nutanix/resources/datasheets/ds-eca-5.15.pdf has a table in the back that compares the delivery methods and shows what is/isn’t included for each.