Seeing as your Smiter will be used only for solo Trist runs, I don't see much of a point to investing much in either of your suggested auras.
Conviction's only real use in Trist is to cancel Meph's. Otherwise you don't benefit much from it.
Salvation is decent but its bonuses dry up with additional points very quickly. As PlayWithHonor suggested, I would say put one point into it and leave it there. If I remember correctly I calculated a 1+15 (from +Skills) to give +106 Res, and 20+15 to give +115. That's 19 Skill points for 9 Res. Of course you won't have +15 without a Torch in your inventory, but the gap still won't change that much. The bonuses from heaping points into Salvation are minimal.
As Dennis said, I would stick with Fanaticism as a main aura, with one-point Salvation for when you need it.