Which train motor is more powerful, 9V or 12V?
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