[Faceman]

a mage/fighter gets to be a fighter who can use wands - period.
If you equip him with a nice suit of elven armor he can also cast some spells but nothing serious.
A fighter/mage however - even if dualed right at the beginning - becomes a proper mage with devastating spell power just like a true mage BUT can have Grandmastery in Staffs, has a better Thac0, can wear Elven Chain, can use swords, ...
With a high level fighter/mage you can just timestop -almost- ANY enemy and then use the attack rounds not to cast but to whack him dead. If you like mages and you ever worried about their low melee abilities the Fighter->Mage is your choice.
If you are looking for an elegant fighter with some magic abilities you could use the Mage->Fighter but I would go for a Blade instead.