What is "stronger"? Does that mean: Who'd beat the other in a one-on-one fight? I guess that question might make sense. But apart from that, I'd simply say they have different strengths. A spell-user have lots of utility spells that are very useful in many situations. Like Teleport. That is in my opinion the main advantage of most pure spell-users. But then again, they have very few skills at decent levels, while a fighter might. And in combat? As long as the spell-user is left undisturbed to cast his spells, he can cause lots of trouble for his opponents, obviously. But he'd HAVE to avoid melee, almost at any cost. The fighters greatest usefulness is, in my opinion, in his ability to stand his ground, and stop and block any foe. And even more so with two weapons, since he can now parry two opponents.
Also, in a party, you'll need versatility. In a party of two, I'd rather have one fighter and one mage, than two pure spell-casters. So, in a sense, I'd say that after the mage already was in the party, the fighter would be better as the next team-member. The team needs balance.
As for causing pain: A hasted fighter with two-weapon combo is... well, trouble. Make him invisible and silent too, and his first round of attack could come as a surprise to an opponent, leaving him unable to parry. I think six attacks (80% + 60% + 60% with two weapons) without getting to parry would cause more harm to a single opponent than almost anything else (except "save-or-die" spells like Absolution, which most decent foes would simply be able to resist). That's how a mage/fighter team could work.