I think David outlines these two things well.
Goals are merely another horizon to define and priorities are a decision you make during the "Do" phase.
I've found the key to making Goals work within GTD is making sure I've defined what's actionable about the goal - i.e. what are next actions or even outcomes to define that help me move closer to the goal. Personally I organize mine in OmniFocus and they are in a separate folder called "Goals".
Priorities by themselves don't work because you really need to consider the other items in the Do phase - context, time available, energy level. In this context the personal payoff decision becomes easier. Not easy but easier.
Just my $0.02.
How did you capture that thought?