Well, i myself usually do the "let him use the artifact on Geralt" choice. Some might argue that dragon scale is better, because you can craft either the dark armor set pants (kinslayer) or the zerrikaternment silver sword, but if you are in all other difficulties other than dark, said set is inaccessible anyway and if you are in dark mode, that silver sword is useless compared to the one from the dark set. Either way, you normally need only 1 dragon scale per game, per run.

Also, doing the reroll, you can exploit a nice trick. By having met certain conditions pre-reroll:
1) Maxing the alchemy talent which amplifies mutagens' effect.
2) Slotting tree's abilities with mutagens (greater ones or madness ones and combinations).

And then *without loading game*, quickly in one sit:
1) Let operator do the reroll.
2) Quickly reopen the previously slotted skills and leave the alchemy ability completely outside (for mutagen effect).

You have fully maxed mutagen effects, without having to use mutagen slotting again anew and they have max potential, without need to even touch that alchemy talent! You can now pick anew the real endgame and really useful tree talents, too!

Staff is not a sword and even if Geralt can use other weapons (like blackjack in beginning and boathook, etc), best is to stick with swords that are far more effective in the hands of a witcher. Alternatively, you can save game and try both options for yourself. Geralt can't use the artifact himself, so that choice also ends in battle. Beware; hardest fight in game, actually!