it's with them both
the diesel you can't tow with due to the dual mass flywheel not liking the starts and being rather more prone to disintegrating due to it being not as highly spec'd due to the torque produced by the engine (or along those tehcniclaly lines)
the st220 you can't tow with due to something to do with the exhaust getting in the way as well as the flywheel issues

obviously these problems can be rectified with the fitment of a "normal" flywheel, but that's looking at a lot of money