Magento 1.9 Checkout – Difference Between Checkout Cart and Checkout Session

If you need data to persist with the shopping cart session, extend the shopping cart. No need to piggyback on the customer persistence.

If you need data, you can extend the following tables - sales_flat_quote, sales_flat_quote_item to inject your domain-specific logic into the sales. The persistence module should resurrect the shopping cart session as per usual - only associating it to customers that have logged in / performed actions on different devices.

Persistance is also limited in CE. Wishlists and more aren't available in CE, only in EE. In CE 1.6 / EE 1.11 it was unable to work with compilation. I'm not sure if this was addressed in later releases.

The code under Mage_Catalog_Block_Product_Abstract has a very useful comment.

/**
 * Retrieve url for add product to cart
 * Will return product view page URL if product has required options
 *
 * @param Mage_Catalog_Model_Product $product
 * @param array $additional
 * @return string
 */
public function getAddToCartUrl($product, $additional = array())
{
    if ($product->getTypeInstance(true)->hasRequiredOptions($product)) {
        if (!isset($additional['_escape'])) {
            $additional['_escape'] = true;
        }
        if (!isset($additional['_query'])) {
            $additional['_query'] = array();
        }
        $additional['_query']['options'] = 'cart';

        return $this->getProductUrl($product, $additional);
    }
    return $this->helper('checkout/cart')->getAddUrl($product, $additional);
}

The difference is that the Mage_Catalog_Block_Product_Abstract::getAddToCartUrl will set the url as the product view page if the product has required options, otherwise it will just return Mage_Checkout_Helper_Cart::getAddUrl.

The getAddToCartUrl can be used when you are not on the product view page, say for the wishlist page. So that you do not get an error when the user tries to add a product to the cart that has required options.