Since no one cares to actually do the math and calculate the extra damage (what a crazy world, in which we verify our claims, huh?!), i will do it: for level 20 mastery, level 20 skeleton mages and compare it to 20/20 raise skeleton.
the formula for the skill level the mages attack has is: skeleton mastery + ((raise skeleton Mage-2) / 2). for 20/20 that would be 20+((20-2)/2)=29
here are charts for their projectile damage: Raise Skeletal Mage - Diablo Wiki
melee skeleton damage: 124-127
Poisen Mage: 727.5 over 4 seconds, 182 over 1 second
their buff was +750% damage and reduce length to 4 seconds. in 2.3 their damage would have been 97 over 300second.
Cold Mage: 107-111
Their buff was damage +50%. 2.3 damage: 71-74, 29 sec frozen/chilled (nightmare cuts duration in 1/2, hell 3/4)
Lightning Mage: 30-205
Buff 5%. 2.3 damage: 29-196
Fire Mage: 121-125
No buff
without auras the damage has been doubled if you pick Skeleton Mages. however, merc might, beast, pride, oath, etc benefit the melee. mages might be a good pick for poor players that want to deal more damage or at ladder reset. considering you already max out mastery, its relatively cheap.
Aura effects:
Merc at lvl 98/Might (lvl 30): Dmg +330%
Pride/Concentration (lvl 16-20): Dmg +285% - 345%
Beast/Fanatiscm (lvl 9): Dmg +95%, AR +80%, IAS +29 (though that is neglegible, since minions have that pause in between attacks)
Oath/Heart of Wolverine (lvl16): Dmg +125%, AR +130%
Total amount of Dmg buff: +895%
on top of that comes amplify damage’s x2 of total damage