Forget the enhancements to graphics and sound - where the 128 version of Gladiator scores over the original is that it is much simpler to play.
The 48K game boasted 25 different moves; however, a third of these required both a shift of a joystick and a double-burst on the joystick. If you didn't have the dexterity of a concert pianist, your early death in the arena was assured. In the 128 game, the moves have been trimmed down to a far more manageable sixteen possible moves and the result is a much more satisfying thrust and parry session.
Another improvement is that you are no longer fighting your identical twin brother. In the original, every Gladiator was the same; now there are two to choose from.
The aim of the game is to win your freedom and to do this there will be a minimum of fourteen fights to win. With each victory you amass more coins but you need 32,400 to become a freeman. Even if you succeed in becoming the Emperor's champion you will still have to gamble your winnings on the outcome of other gladiator's bouts. The gambling element may add an extra element to the game but some may find it a letdown. If you're just risked your three lives in the arena, then risking your shirt as a spectator just doesn't have the same appeal. If you are a better gambler than a fighter and pick up sufficient coins, you can buy your certificate of freedom and complete the game.
Overall, Gladiator 128 is a modest advance on the original and the simplified fighting moves may mean that you could make Emperor's champion status far faster.