[SalesForce] How to specify the difference between client controller vs server controller action in Lightning

I think, it is just not recommended, because if you go further ahead and try the following in your example:

Component

<aura:component controller="MyController">
    <ui:button label="Test" press="{!c.start}" />
</aura:component>

JS Controller

 start : function(component, event, helper) {
    helper.start(component);
 },
 test : function(component, event, helper) {
    console.log('in Js controller method') ;
    return 'xyz'
 }

JS Helper

start : function(component, event, helper) {
        var action = component.get('c.test');
        action.setCallback(this, function(resp) {
            var state= resp.getState();
            console.log('IN CALLBACK; STATE='+state);
            console.log('IN CALLBACK; Response='+state);
        });

        $A.enqueueAction(action);
}

And do not modify anything in the apex class.Once you click on the button you can see the following in log :

in Js controller method

IN CALLBACK; STATE="SUCCESS"

IN CALLBACK; Response="xyz"

So, the code does not break and runs fine, but at the same time it doesn't behave as expected. component.get('c.test'); statement returns a reference to JS contorller's action 'test' instead of the apex action.

One way to return an array of objects where the object fields have the keys of "value" and "label" as required by your component is:

@AuraEnabled
public static List<Map<String, String>> getRecordTypes() {

    System.debug('getRecordTypes starting...');

    List<Map<String, String>> items = new List<Map<String, String>>();
    for (RecordType rt : [
            SELECT id, name
            FROM RecordType
            WHERE SobjectType = 'IP__c'
            ]) {
        items.add(new Map<String, String>{'value' => rt.Id, 'label' => rt.Name});
    }
    return items;
}

Or if you are doing a lot of this, you can create your own class as described in Why is SelectOption not supported by JSON class?.

PS

This markup to match this is:

<option text="{!item.label}" value="{!item.value}"/>