m/k has more PvP oriented/useful skills than m/w(pve dominant mage). while all mages are decent at PvP to begin with(kinda like rogues and warden), m/k gets enhanced armor and a spammable instant, among other things(including a nifty "oh, crap" skill)...m/w doesn't have much in the way of PvP...
however, if u are m/p, that would still be a better combo for PvP than m/k IMO, but at that point it's more personal pref(survivability due to defense or heals/immune)