The only thing that matters in a fight now is speed, which Sabin wins on.
Are we really going to suggest that speed stats matter in a grudge match? I already addressed this above. Besides the difference between their speed is only 4 points (as compared to the 11-point difference between their magic power) and speed stats can be, and probably have been, modified with espers. It is trivial to make up a 4-point speed difference. A 11-point magic power difference is a bit more difficult to make up.
Furthermore, even if we assume speed stats
do matter, we also have to assume the amount of time attacks take to pull off matters too. Even assuming a small difference in speed, Terra can swing the Ultima Weapon and she will still be finished attacking before Sabin can pull off one of his blitzes. The moment she swings the Ultima Weapon this thing is over. It is unblockable, and assuming the characters are anywhere near their peak prowess it will deal 9,999 damage. It doesn't matter what Sabin is in the middle of doing. He will be knocked out instantly.
The worst that Terra can suffer is that this ends up as a draw, with each knocking the other out at the same time.