Change Default Content when Creating a Post based on Previous Category Choice

Question

I'm working on a system that forces users (unfamiliar with wp) to choose a category before creating a new post, and then adds some appropriate html layout styling to help them.

The following code works if they choose category "x". How can it be adapted to cope with a couple of alternative category situations?

Human Code Required:

• If the new post is category X: add this html.
• If the new post is category Y: add this html instead.
• If the new post is any other category: add this html.

Existing Code - Inserts some html to new posts if the category is "X":

// This function will only be called when creating an empty post,
// via `get_default_post_to_edit()`, called in post-new.php
function wpse14403_wp_insert_post( $post_id )
{
    wp_set_post_categories( $post_id, $_REQUEST['category_id'] );
}

add_filter( 'default_content', 'wpa70073_default_products_content' );

function wpa70073_default_products_content( $content ) {

    // change this to your desired category ID
    $products_category_id = 4;

    if( 
        isset( $_REQUEST['category_id'][0] )
        && $products_category_id == $_REQUEST['category_id'][0] 
        )
        return 'some html styling';
}

UPDATE: Tried adding this to the end, but it didn't work...

// change this to your desired category ID

$products_category_id = 4;................

    if( 
        isset( $_REQUEST['category_id'][0] )
        && $products_category_id == $_REQUEST['category_id'][0] 
        )
        return 'Some html styling';

    if ( $products_category_id = '6' ) {
        return 'Some alternative html styling';
    } else {
        return 'Some default html styling';
    }
}

Also tried editing the code along the lines of the cpt thread suggested below, but again no luck :(

add_filter( 'default_content', 'wpa70073_default_products_content' );
function wpa70073_default_products_content( $content ) {
    switch($content->category){
        // change this to your desired category ID
        case'4':
            $content='cat 4 content';
            break;
        case'6':
            $content='news content';
            break;
        default:
            $content='default content';
            break;
    }
    return $content;
}

Answer 1

You should have linked to the original Q&A since the beginning...
Another great code from @JanFabry, btw :)

Force category choice before creating new post?

Using exactly the code provided in his answer, this is how to make the filter default_content work:

add_filter( 'default_content', 'wpse_71772_default_content', 10, 2 );

function wpse_71772_default_content( $content, $post ) 
{
    if( !isset( $_REQUEST['category_id'] ) )
        return $content;

    switch( $_REQUEST['category_id'][0] ){
        // change this to your desired category ID
        case'1':
            $content='cat 4 content';
            break;
        case'3':
            $content='news content';
            break;
        default:
            $content='default content';
            break;
    }
    return $content;
}

Answer 2

Considering the nature of filters wouldn't it be better to 'filter' the output rather then writing this to the database?

With your approach changing the post two years from now after you've generated thousands of items in the various categories will be a major pain. But filtering the output would be as simple as changing a line or two of code.

Here's a link to a very similar question Default or Preset Content for Custom Post Types