My copy of Mekton II arrived yesterday in the mail, and I spent much of the evening lightly reading it.
I can see why people prefer Mekton Z, but Mekton II has a number of flavorful touches I don't recall seeing in MTZ. Mekton II's production is very "early 90's, late 80's" in style, with primitive artwork and tables, and a somewhat scattered layout
For example, there is a table for Mekton weapons that miss to show what sort of collateral damage they would do to a city - crushed facades, melted steel, craters in the road, etc. Mekton II seems to have a lot more of these "flavor" tables than Mekton Zeta does, though again, MTZ is a more dense manuscript, and I haven't yet gone looking.
Mekton II and Mekton Zeta are both implementations of the "Interlok" gaming system, which has a variety of stats, some physical, some mental and some social. The stats themselves are mostly compatible between the games, and some exist to provide flavor, like "Cool", which is naturally enough, how cool you are. All values range from 1-10, and d10 is the primary dice used. Tasks are usually opposed d10+Stat+Skill rolls, or in the case of what d20 would call "Difficulty Class" ratings, Difficulty+d10. MTZ does away with the "+d10" for DC checks and rates its DC's as 5 points higher than Mekton II. I think I like the Mekton II way, but I can see the advantages of a fixed target number.
Once Mekton Zeta-ism I do intend to use is the "computed stat" for "Stability", which is Cool*2.5, and gives the target number for overcoming someone in a social situation, such as asking someone out on a date or trying to interrogate someone. I didn't see a "Resist Seduction" or similar skill (though one is mentioned in the Mekton II contested tasks example), and "Stability" would make for a nice substitute. To "Mekton II-ify" the rule, I might take (Cool*2.5 - 5)+d10, though in my experience players get itchy with that much math, even when they only have to compute it once.
M2's internal organization definitely shows its early 90's roots. Artwork is primitive, fonts and layout are simple, and organization is a bit confusing. It doesn't help that the examples frequently use skills that do not exist, though I expect the author did this to try to convey the free-wheeling spirit of the game.
Combat is broadly the same, but differs in some small details, and is quite dangerous in both games unless you are wearing some armor or only engaging in fist fights. Mekton II seems to have more in the way of kung-fu options for both humans and Mecha, which is amusing. If your gun can't punch through your enemy's armor, maybe you can roundhouse kick his mech in the head.
Character generation is similar in both games, but Mekton Zeta adds the Traveller-like ability to "serves some terms" to get more skills, and you can either play a "Rising Star" or and "Experienced Hand". I would probably use this same rule in Mekton II if I were running a campaign; there's nothing inherently "Zeta" about it.
All in all, I'm left with the impression that Mekton II is sort of like AD&D 2nd Edition, whereas Mekton Zeta is sort of like D&D 3.0. Mekton II is idiosyncratic and hard to follow in some places, but has a bit more character in the main rule book. Mekton Zeta is a more solid gaming system, but it loses a bit of flavor in its attempt to streamline and rationalize the Mekton system.
Which would I use to run a game? If I were running Operation: Rimfire, I think I'd use Mekton II, since that's what the adventure was designed for. If I were doing a new adventure, I would seriously consider using Mekton Zeta in the Mekton Empires setting, though Empires would need some conversion work.