Lots of people recommending the transforce and without pissing them off I have to disagree that it is a good trailer tire. IMO you want a "rib" design tire(r250, xps etc.). It has a better tread pattern for trailer use, including a closed shoulder they are also all steel. They move laterally better than tires designed for traction positions among other things.
But given the choice I would go with the transforce over an ST tire in most situations (6k axles). If you are a short tripper, less than 75 miles and don't load to heavy then Maxxis ST's might be a reasonable alternative.