What is the difference in meaning/usage between "vorhersagen" and "voraussagen" ?
Is one more commonly used than the other?
"Voraussagen" is a little bit more metaphysical than "vorhersagen". Similar to "prophecy" vs "prediction".
You would "voraussagen" the future but you would "vorhersagen" the weather.
There's no difference whatsoever. Vorhersagen is a little more common than voraussagen, not least because the weather forecast is usually Wettervorhersage. It's possible that there are further common collocations, but I wouldn't come up with any off the top of my head.
"voraussagen" is forecasting. A forecast tries to predict the future by extrapolating current data.
"vorhersagen" is foretelling. Foretelling means telling someone the future without having clear data, or otherwise having obtained information by somewhat "occult" means.
Basically the difference between "voraussagen" and "vorhersagen", is the difference between a stock market analyst and an oracle.