If the scope is only being used as a guide scope, go for the cheaper option (Orion). if you want to have the option of using the guidescope for photography (and the main scope as a guidescope), go with the ED80.
Rob,
I agree with Geoff.
I have a ST80 with the QHY5 guide set-up and it works 100%. Short, lightweight and easy to use.
I also have the ED80, which is a very good scope, but much heavier and bulkier than the ST80.