Emily Seymour, Which? energy editor, says...Prepayment unit prices are a little cheaper than the price-capped variable tariffs for direct debit customers at the moment.Prepayment prices used to always be more expensive than paying by direct debit to account for higher admin costs, but the government pledged to scrap the extra charges for prepayment meter customers and prices are now more balanced.
Most notably, this price difference is only for customers paying a price-capped variable rate - as soon as you look at fixed deals, direct debit customers tend to have access to cheaper options, with fewer available for prepayment customers. Next, prepayment meters aren't as convenient - you will need to remember to physically top up your meter when your credit runs low.