mmmmm... I'd say bus,
Continental-Auto from Avenida de America. I have done the trip often during winter, and the chances of a serious blizzard exist, but it's a matter of luck.
I've done the trip during jan-feb dozens of times and I only have had slight snow troubles a couple of times.
The rest of the time it has been riding trough the Madrid sierra under the sun and into the inmaculate white snow... a priceless experience. Imagine the Velazquez blue skies above and the snow white below... magnificent!
I mean, this is not Finland or Minnessota, it can snow heavily for some hours and then everything goes back to normal. This happens regularly every winter from northern Madrid to the northen Spain mountains, but road traffic is only slightly distorted.
And man, Continental Auto drivers are very efficient in winter driving!
A serious blizzard that traps you let's say for a couple of days or three is very uncommon, only once in a few years.
The worst thing that could happen normaly is a couple of hours delay.
For example: last March all the buses heading north from Madrid were trapped all night by a blizzard in the Burgos province. Sports pavilions were enabled for the troubled passengers to spend the night there.
This was the kind of heavy blizzard that you must be very unlucky to encounter.
Go for the bus!
No pasa nada!