Thanks all for your suggestions.
In the end, I decided to dual at level 10 instead because...
In TOB, in addition to those great level 10 spells, I think other special abilities the mage gains include additional 6th, 7th and 8th level spells. So the lower you dual, the more additional spells you can gain later. As a mage, I prefer to specialise in what I'm supposed to be good at, so decided to forego fighting abilities in favour of spellcasting ones.