Which train motor is more powerful, 9V or 12V? I recently picked up LEGO bricks again and due to lack of space I'm planning to build a multilevel train layout.
I do have a descent amount of 12V track along with trains and switch points, but I was wondering if it would be a good idea to switch to 9V.

Because there will be a lot of inclined track on this layout I was wondering which train-motor-units are more powerful.
The ones I have, 12V from the gray era, or the track-powered 9 volt ones?

12 volt:


9 volt:
A user on YouTube posted the following video that tests a lot of the various train motors in Lego history for their pulling strength.

The battery model train from the 1960's destroyed the competition.

Pulling Power of LEGO Trains 1966 till 2009 (Systems: 4.5V 12V 9V IR)

The newer engines do not have rubber traction to grip, so they have a tendency to slip more.  The lack of weight on top of the engines also hindered some of its ability.  The newer engines tend to be faster, but not more powerful.

Another link that summarizes many of the various Lego motors (unfortunately, it didn't include the 12v line):

LEGO® 9V Technic Motors compared characteristics