Use it like this
<?php if ( in_category( '951' )) {
echo 'Online. Todas las provincias'; }
else {
global $wp_query;
$postid = $wp_query->post->ID;
$meta = get_post_meta($postid, 'provinciadestacada', true);
echo $meta;
}
?>
To do it in reverse as you want:
global $wp_query;
$postid = $wp_query->post->ID;
$meta = get_post_meta($postid, 'provinciadestacada', true);
if(!empty($meta)) {
echo $meta;
}
elseif(in_category( '951' )) {
echo 'Todas las provincias';
}