Polearms are varied. With polearm wpf you can bring the best shield crusher, the best anticav weapon, the best tincan opener, the best horseback weapon etc.
2h on the other hand are not that varied. You have a crapload of swords (each with it's subtle pro's and con's), a few axes and a few maces. As you might have guessed, the swords are the best choice.
But the problem of polearms is that all these weapons are themselves very specialized. You can't break shields with a pike or go on horseback with a bec (okay it's possible but hardly viable). The DGS and GGS don't have these problems. They excel at duelling and are good for everything else, be it anticav, tincan opening or fighting shielders.
That's why 2h allmost never take anything more than one weapon, and polearmers usually take two.
I will not risk myself at comparing the damage and speeds of 2h and polearms. You can see everything and the contrary happen due to weapon and build diversity so I'd say they are globally the same.
The ability of polearms (sometimes 2h do that too) to protect teammates from your own overhead slashes is balanced with the fact that side swings of 2h go through teammates way easier than polearm side swings (animations).