If you were going to take a warding stave anyway, give it to the character. As noted, there is nothing like a 2++ save challenger to annoy character monstrous creatures (wraithlord, tervigon, bloodthirster et al).
As far as the free stuff goes...the standard argument in favour of power fists is that whilst you run the risk of being killed before you strike in a challenge, if you do hit you'll flatten most 'normal' multiwound HQ characters - in a single blow, whilst a guy with a power sword will only likely do one wound, which isn't enough. The problem with this theory is that it doesn't work for knights - since everything's a force weapon, one wound is enough, it doesn't matter what weapon it's from.
The halberd is probably the best general-purpose weapon for duels. The extra invulnerable save for the sword only matters in close combat, and only matters against AP2 weapons, and - whilst an improved invulnerable is nice, even a 4++ still only works half the time. The best protection is for your opponent to be dead before it's their go, and hence (for me) halberds are the way to go for terminator justicars as well if you don't want to take a stave.
That choice depends largely on how many AP2 close combat weapons you expect and whether you have the points. Versus a marine army, probably okay. Versus tyranids, for example, think very carefully (especially since they have stuff that will strike even before a guy with a halberd).
