In general I think the Duratec DOHC engines are more reliable,
definitely more fun to drive (or able, highway, etc)
I have tauruses with both engines sitting in my driveway.
I also had a 94 Vulcan that I drove for 276K before it was wrecked.
They are both very good engines.
THe Vulcan has proven itself for years and is one of the most reliable and problem free engines that Ford has ever done.
The OHV (vulcan) has more bottom end torque but is easily outran by the Duratec after it gets moving.
Don't buy one unless it has the Duratec (DOHC). You will understand after you have driven both.
Mike