Hi Andrew,
http://www.mojoportal.com/using-paypal-website-payments-pro.aspx
In order to use PayPalDirect, it is required to also support PayPalExpress. This is implemented in mojoPortal when using PayPalDirect, but it is the least tested scenario, I'm not using it and don't know of anyone using it so your mileage may vary. I don't think it was implemented to be used by itself but only to fulfill the requirement that it be available if using PayPalDirect.
I recommend stick with PayPal Standard.
Best,
Joe