The roads are not the danger, the drivers on the road are the danger!
Think about your question inversely (antonym); "Which is safer, icy roads or icy roads with snow on top?"
It is not the presence or lack of ice/snow that creates the danger/safety, it is how the driver responds to the presence of the snow/ice. For example, drivers with 4x4 trucks occasionally believe the four wheel drive makes driving on snow and ice safer, and it does... Except when the 4x4 driver starts driving too fast for conditions, and crashes.
The biggest danger is not adjusting your driving for conditions.
Which leads us to black ice this ice can develop on roads and be difficult or impossible to see. Any road condition that is not apparent to the driver is going to be the most dangerous.
So given your question, (black) ice is more dangerous than (white) snow. But even this can be anticipated and adjusted for. If there is moisture and road temperatures below freezing adjust your driving for slippery roads.
Lastly, Aquaplaning or hydroplaning is just as dangerous in warm weather as ice and snow in cold weather.