so far I know (correct if I'm wrong):

 

1. kinship ring (berserker) works on spear;

2. the only difference between spear & 2h sword is attack style (stab vs. slash);

3. more boss are weak to stab than to slash.

 

In that case, spear is superior to 2h sword, right?

Yes.

The one floor I'd draw:

 

In EoC all the bosses are 'no weakness' and their old weaknesses do not appear to have carried over at all. So spear and 2h are perfectly matched with maul right up there too.

I think it's just down to the most common weakness spread across all the monsters in dungeons since all 2H weapons of the same tier are matched power-wise. I'd suggest getting a primal maul if you have 99 strength, since that's the earliest primal 2H you can get. I honestly prefer magic because it has 4 different styles that have to be balanced among all the monster types, while melee only has 3(To a lesser extent, range has 1. Very under-powered in DG unless you have a hex). So i think on an average you come across more mage-weak enemies.