meteor collection helpers vs orm

I would like some kind of Model for my Meteor collections. The two options I could find for this would be to use collection helpers:

Users = new Mongo.Collection('users')
Users.helpers({
  fullName() {
    return `${this.firstName} ${this.lastName}`
  }
})

or to map the returned objects to class instances:

 class UserCollection extends Mongo.Collection {
    fetch() : User[] {
        return super.fetch().map(user => new User(user))
    }
}

let users = new UserCollection('users')

class User {
    collection: Mongo.Collection
    _id: string
    firstName: string
    lastName: string

    constructor({firstName: string, lastName: string) {
        this.firstName = firstName
        this.lastName = lastName
        this.collection = users
    }

    get fullname () {
        return `${this.firstName} ${this.lastName}`
    }
    save () {
        if(this._id) {
            this.collection.update({ $set: { ...this }})
        }
        else {
            this.collection.insert({...this})
        }
    }
}

Which variant is preferable? I like B more, but rarely found any examples using it.