I'm a PayPal client for over ten years and can't imagine switching to another system - I haven't actually heard of RBS WorldPay so I Googled for
reviews and came up with some pretty negative information (granted, there are plenty of negative PayPal reviews out there as well so I don't know if this is really going to help answer your concerns and questions)
I'm curious to know if you've signed up for their service since you posted this and if so, what your experience has been?