Alertpay is incredibly professional and supports a wider array of countries. It really is beyond me why not many people use it. Perhaps because Paypal is backed by Ebay. AP doesn't have a e-commerce giant backing it up.
Paypal is quite common, but if I were to choose between the two, I'd pick Alertpay. To someone who said that Paypal only blocks kids - that's not entirely true. I'm above 18, I had a business verified Paypal account and one day they just decided to up and limit my account. After providing them with the docs, they refused to restore my account status and told me they'd keep the money for 6 months. I called them and they said they found my account to be of a risk level they were not comfortable with. Great reasoning, isn't it?