When it comes to Mektons, IMHO, the combats are definitely faster and less fiddly for both. However, once you expand into the Technical Manuals (separate books) the design process can become very fiddly because you can absolutely nail down what you imagine for the mecha with the system.

Both Mekton II and Mekton Zeta play equally well, but the mecha in II are more fragile than the same size mecha in Zeta. Zeta also has base rules for Roadstrikers, Mektons, and Starships in the main book while II has them in three separate books. Overall, it will depend on what you use it for, but the II core rules is a more self contained ruleset while the Zeta core rules is a more versatile ruleset.

If you are introducing new players to the system, Mekton II may be preferable to Mekton Zeta because it is more self contained and thus easier to grasp.