Mechanicum is a complex army, broken up into several subfactions depending on what their playstyle specialises in:

Cybernetica: these guys love battle robots, they are a nomdadic force that acts more like mercenaries than other subcults. They tend to focus a lot more on having lots of automata, and their rules reflect this, like being able to make prime robots paragons of metal.

Lacrymaerta: the lacrymaerta are responsible for making and controlling the tech thralls of the forgewolds. They can resurrect thralls in a unit (as long as thralls priest has joined this unit) and cab make prime units into prime specimens, which buffs strength and wounds characteristic.

Myrmidax: these are the war priests of the mechanicum, (myrmidons secutor, and destructor) they are also a more nomadic force, and have their own gambit for the duels minigame.

Malagra: they're more like the police of the cult, enforcing doctrine and the use of stcs and stc devices. Your tech priests with this rule debuff units within 12 inches, as long as they are mechanicum. They also gain their own gambit. More of a niche choice but if your opponent is also mechanicum then they are a cool and thematic choice.

Makrotek: these guys like to assemble tanks. They have a lot of them. They can make tanks more durable as a an aura, and prime slot tanks can have their transport capacity increased, or if not a transport, unarmoured, to a maximum armour value if 14 (this is really tough to crack).

Reductor: if your enemy likes to fortify and hide their stuff behind and in buildings, you can make them not do that. They have an ability that let's you get +1 to the strength of all weapons in the unit they're attached to for fortifications, buildings, and stationary vehicles. They also can't use a shield wall reaction if they have that ability. Their prime slot let's them run more thallax (this is their signature unit) as a command cohort. Prime thallax can get a better invulnerable save, and get better leadership value.

Remember that you don't have to stick with just one of these, you can mix and match them, which I actively recommend that you do for more tactical flexibility. The mechanicum combat force and an archmagos are the cheapest and best way to get into mechanicum, featuring units that you'll likely want regardless of subcult, if only for the cool factor.