I just read a very good comparison on a trailer manufacturer's site a few days ago. They make and sell both, so I trust their information. I think much of what they said has been repeated above. They also mentioned parking or storing on uneven ground would be bad for torsion. Overall, my take on it was that springs are better for heavier use and abuse, and will likely last longer with lower cost to maintain and repair. The advantage of torsion is that they are low, and provide a nice ride.