You can use both if you like, but that just creates confusion for your customers when they see two PayPal options and don't know what to choose. If a customer has to think "um, which one?", you slow them down during the most important phase of shopping: checkout/payment. Too many choices can cause them to stall, which could cause them to get sidetracked, and you lose a sale.
Express Checkout lets them accelerate through checkout slightly faster and is somewhat more reliable than the IPN method.
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