Plinian eruptions on both, pyroclastic flows. Not much to choose, both have had explosive, cataclysmic eruptions. I remember the St Helens eruption in 1980. The mountain developed a huge bulge which was blown away, blasting the mountain apart. Something similar happened to Vesuvius in the AD79 eruption. Of the two, I would say Vesuvius is the more dangerous. St Helens is more destructive, but millions of people live in the shadow of Vesuvius and in Naples bay, and Vesuvius is always ready to erupt. Before the last eruption in 1944, which wiped out a USAF base, Vesuvius had been constantly active for at least 500 years, with no quiet spell longer than 7.5 years. She is long overdue an eruption. The thing is that something like spewing ash from Vesuvius is potentially far more destructive than spewing ash from St Helens, with far more people crammed onto its slopes.