The Roman Gladius was effectively used for centuries and during that time saw few changes in design, but the ones it did see were significant. The Maintz pattern being an evolution of the spatha and the Pompeii evolving further from the Maintz pattern. The most easily identified differences are the body style of the blade and the point of the sword.
Right from the beginning a person can see that the body of these two styles vary from each other. The body or waist of the Maintz pattern gladius blade is concave on both sides, also known as wasp-waisted. This was taken from the spatha and added extra cutting power to chops and slashes. After years of use and armor improvements, the Pompeii blade was put into use which eliminated this curvature. By making the Pompeii's waist straight, it added strength behind the point making it a stronger thrusting weapon.
The change in the body also had ties to the change in the tip. The Maintz pattern had a longer and therefore more narrow point. While still effective at thrusts and stabs, this did not lend any strength to the tip. The Pompeii shortened this point and combined with the strength behind the straight blade, made for a weapon that was superior and stronger for thrusts. Being that the Romans found the straight thrust as the most effective and defensible attack, it made clear sense to evolve the gladius in this way.
Of course, you don't have to take my word for it. You can find both styles of this sword on sale at Museum Replicas, where you can make your own comparisons on this historic short sword!