PDT is not guaranteed to happen, it only happens if the user clicks the link to return to your site from paypal, with PayPal Standard the only way to get a for sure post from PayPal is with IPN, so you should definitely configure IPN if using Standard.
I do think PayPal Express is probably the way to go since it does get immediate completion of the order and doesn't require configuring special urls like PDT or IPN. One of the problems with PayPal Standard is that you can only configure one PDT and IPN url per account which makes it difficult to use the same account on multiple sites whereas there is no such problem using a single paypal account with multiple sites using Express since there is no site specific url to configure in the paypal account settings.
I agree there should be a way to delete old carts, I've logged that as a to do in our project tracker so we will get that implemented in the next update for Site Membership Pro.
Hope that helps,