How to add an alternative style sheet as a theme option?

I’m trying to create a few extra theme options in the “customise” section of the wp admin menu. I would like to offer an option to load an alternative style sheet.

Question: How can I adapt the following code to replace the stylesheet name “posts.css” with a user-selected option?

function theme_styles()  
{ 
 // Register the style like this for a theme:  
 // (First the unique name for the style (custom-style) then the src, 
 // then dependencies and ver no. and media type)
 wp_register_style( 'custom-style', 
get_template_directory_uri() . '/posts.css', 
array(), 
'20120208', 
'all' );

// enqueing:
wp_enqueue_style( 'custom-style' );
}
add_action('wp_enqueue_scripts', 'theme_styles');

This is the particular line of code I am having trouble with – I have tried this, where ‘posts’ is a theme option and ‘style.css’ is a default – but it gives a blank page:

get_template_directory_uri() . '/<?php echo get_theme_mod( 'posts', 'style.css' ); ?>', 

And this is how I added the alternative stylesheet option to the wp menu incase it matters:

$wp_customize->add_setting( 'posts', array(
    'default'        => 'style.css',
) );

$wp_customize->add_control( 'posts', array(
    'label'   => 'Post Blocks',
    'section' => 'themedemo_demo_settings',
'type'       => 'select',
'choices'    => array(
    'style.css' => 'Default',
    'posts.css' => 'No blocks',
            ),
) );

Thanks