Most Efficient One-To-Many Relationships in Google App Engine Datastore?

Sorry if this question is too simple; I'm only entering 9th grade.

I'm trying to learn about NoSQL database design. I want to design a Google Datastore model that minimizes the number of read/writes.

Here is a toy example for a blog post and comments in a one-to-many relationship. Which is more efficient - storing all of the comments in a StructuredProperty or using a KeyProperty in the Comment model?

Again, the objective is to minimize the number of read/writes to the datastore. You may make the following assumptions:

• Comments will not be retrieved independently of their respective blog post. (I suspect that this makes the StructuredProperty most preferable.)
• Comments will need to be sortable by date, rating, author, etc. (Subproperties in the datastore cannot be indexed, so perhaps this could affect performance?)
• Both blog posts and comments may be edited (or even deleted) after they are created.

Using StructuredProperty:

from google.appengine.ext import ndb

class Comment(ndb.Model):
    various properties...

class BlogPost(ndb.Model):
    comments = ndb.StructuredProperty(Comment, repeated=True)
    various other properties...

Using KeyProperty:

from google.appengine.ext import ndb

class BlogPost(ndb.Model):
    various properties...

class Comment(ndb.Model):
    blogPost = ndb.KeyProperty(kind=BlogPost)
    various other properties...

Feel free to bring up any other considerations that relate to efficiently representing a one-to-many relationship with regards to minimizing the number of read/writes to the datastore.

Thanks.