I found myself reliably CT with a Mallet when I had 24 STR.
I found myself reliably CT with a GM when I had 21 STR.
The Mallet is longer and faster, so it becomes better than the GM the more slower you get. At 24, they are equally good, at 27 Mallet is better because of the extra reach and speed, and at 21 GM is better since the Mallet doesn't CT on most things.
Note: The Mallet doesn't CT on everything at 24 STR. Huscarl and steel shield will stop it, but all weapons and most of the shields it will CT on, unless you are getting negative speed bonus.