The Kensai is a nasty piece of work. The wizardslayer is a nasty piece of shit
No really, the wizardslayer can't use any magical items, so that means no rings, cloaks, belts, etc. I'm not sure about potions either.
The Kensai however can't use ranged weapons, but can use throwing axes, and can't use armor of any kind, meaning no shields, plates or helmets.
I'd go with the Kensai. The kai special ability kicks ass when you need it to.
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Why?