ags is alot cheaper, 1100g actually, and the 1875 recipe for refreshers is alot, compared to what else he needs to buy.
but the oblivion staff can be built from the side shops and gives decent mana regen.
either or, doesnt really matter
the refresher route just costs alot of mana, spending 875 mana on dropping 2 infernals, when you only have +-1100 does make managing your mana, difficult.
the ags gives you more bang for your buck, without destroying your mana pool, and the bonus +-400HP and is great.