Which is the better design for this API response - performance

Question

I'm trying to decide upon the best format of response for my API. I need to return a reports response which provides information on the report itself and the fields contained on it. Fields can be of differing types, so there can be: SelectList; TextArea; Location etc..
They each use different properties, so "SelectList" might use "Value" to store its string value and "Location" might use "ChildItems" to hold "Longitude" "Latitude" etc.
Here's what I mean:
"ReportList": [
{
"Fields": [
{
"Id": {},
"Label": "",
"Value": "",
"FieldType": "",
"FieldBankFieldId": {},
"ChildItems": [
{
"Item": "",
"Value": ""
}
]
}
]
}
The problem with this is I'm expecting the users to know when a value is supposed to be null. So I'm expecting a person looking to extract the value from "Location" to extract it from "ChildItems" and not "Value". The benefit to this however, is it's much easier to query for things than the alternative which is the following:
"ReportList": [
{
"Fields": [
{
"SelectList": [
{
"Id": {},
"Label": "",
"Value": "",
}
]
"Location": [
{
"Id": {},
"Label": "",
"Latitude": "",
"Longitude": "",
"etc": "",
}
]
}
]
}
So this one is a reports list that contains a list of fields which on it contains a list of fieldtype for every fieldtype I have (15 or something like that). This is opposed to just having a list of reports which has a list of fields with a "fieldtype" enum which I think is fairly easy to manipulate.
So the Question: Which format is best for a response? Any alternatives and comments appreciated.
EDIT:
To query all fields by fieldtype in a report and get values with the first way it would go something like this:
foreach(field in fields)
{
switch(field.fieldType){
case FieldType.Location :
var locationValue = field.childitems;
break;
case FieldType.SelectList:
var valueselectlist = field.Value;
break;
}
The second one would be like:
foreach(field in fields)
{
foreach(location in field.Locations)
{
var latitude = location.Latitude;
}
foreach(selectList in field.SelectLists)
{
var value= selectList.Value;
}
}
I think the right answer is the first one. With the switch statement. It makes it easier to query on for things like: Get me the value of the field with the id of this guid. It just means putting it through a big switch statement.

Answer 1

I went with the first one because It's easier to query for the most common use case. I'll expect the client code to put it into their own schema if they want to change it.