Question about weapons and damge dealing

Hi all. I'm trying to make a good assassin and I was looking to acquire a good weapon, but I have a couple of doubts:

1) I noticed that some assassin daggers deal piercing damage and others slashing, is there any difference?
I mean, excluding the use of abilities that prevent some tipe of damage, piercing is more effective or this is just a useless difference?

2) The weapon ability request affects the damage?
If I have 16 in dagger mastery is there any difference in using a req 9 or a req 13 dagger?

I know this may seem noob question but I've always played with an ele and for me all this discussion about the damage of the weapon was useless, but now i'd like to have some advices.
Thanks to all in advance

if you are running 16 in dagger mastery then there is no difference in using a req 13 or a req 9 max dmg weapon....

most mele weapons do slashing type of damage...although there are a few that don't but then again there are several items that provide protection against that specific dmg type....

As Coridan said, there is no difference in your ability to use one or the other if you have a 16. As far as piercing vs slashing, there are several skills, armors in the game that reduce damage from piercing, however, there is only one skill that affects slashing damage, it's a paragon skill and it doesn't see much use.