Yes, you want slow/slow. Your strikes are based on a percentage of the combined total weapon damage. Slower weapons generally hit harder. (If they don't, then their DpS is WAY lower than the fast weapon). So, unless the average damage of a faster weapon is higher, stick with the slow weapon.
And I'm thinking Cookie's Tenderizer is the best Heroic dungeon weapon avaialble for DW Frost, but I just looked at a list of weapons on WoWhead with a H dungeon and speed >2.5 filter. It may have left something out. Or there may be something with super optimal stats.
As it stands, EJ seems to be telling me that you should reforge the mastery off of the tenderizer in favor of expertise (assuming you won't cap it early).
Anyways, you all are horrible, @#%^ed up people