I think that depends very much on you. You will be riding, you will have to follow the route somehow. If you're not used to a GPS, then try following the route sheet, as that's presumably what you did in the past. I find a GPS easier to follow but it doesn't seem worth buying one specially for what might be a one-off event. OTOH, if you use a GPS anyway, it would be counter-intuitive at best not to use it for an audax too.