Clarification:
What you refer to as "pp pro" is probably PayPal Website Payments Pro -- which is a credit card processing gateway.
Iridium is a credit card processing gateway.
PayPal Payflow Pro is a credit card processing gateway.
All the above gateways keep the customer on your site while they enter their payment information.
PayPal Express Checkout and PayPal Website Payments Standard allow customers to checkout using their own PayPal funds or their bank account funds, or a credit card, but they temporarily leave your site to provide payment details, and then return when finished.
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