Well thats a good question. In general, the condition that the plates have exactly the same density structure or mean density is intrinsically very very rare. Consider that we refer to the mean density of the plate...that means the average density from the surface down to about 70-100 km...So given all the irregularities (the earth is heterogeneous at small scale) that exist it will be EXTREMELY rare that two plate will have the same density. I cannot think of a case where this has happened.
Basically the age of the two plates that form a boundary will differ and density correlates roughly with age... so as long as the ages are different so will the densities.
If they have equal density, good question - it probably depends on the geometry of the collision.
However, it really depends upon the density of the plate relative to the density of the mantle underneath. Continental plates are less dense, because they're lighter material, and when they collide, they bunch up and form mountains (e.g. the Himalaya). When ocean plates collide, ONE of them WILL go down, and you get island arcs. There are a number of these in the Pacific: Melanesia, the Marianas, Phillipenes, Japan, the Aleutians - all of these are created by one oceanic plate being subducting underneath another and plunging down into the mantle. There is one in the Carribean as well - this is what caused the recent earthquake in Haiti. Ocean crust is denser than the mantle because it's made of the same stuff but is colder.