Foebane is 2d4 +5, +1d4, +6 at its' very best.
This will give you an average of 18.5 damage.
For comparison, the Purifier is 2d4 +5, +5 at its' very best.
This will give you an average of 15 damage. Purifier works better as an off-hand weapon though, unless you want to combine it with the Defender of Easthaven. In which case, you get your Magic Resistance and Physical Resistance up the wazoo.
Of course, things get trickier when you factor in Critical Hits.