It depends on what you're crafting, what kind of parts you're crafting it from, and what kind of labour is involved. If you're assembling a weapon from a pile of blessed components in accordance with Adeptus Mechanicus scriptures, Tech-use might be more appropriate; If you're bolting leather plates onto a soldered chain undermantle using a heated iron rod as your adopted father taught you to do on the hive world you grew up on, armourer might be more your thing.
There are sometimes grey areas, where two skills seem like they could both apply. In such cases, a GM ruling is required; Your GM may decide that one skill is more appropriate than the other, that you can pick which skill to use, that both are necessary and that you should roll whichever you're worse at, or something even more exotic. Depending on your GM, you may or may not be able to negotiate by suggesting that you'll take an approach that favours the skill you prefer, or by reading out the specifics of the skill description and explaining how they relate to the task you're attempting.
In other words, the skill required for a task depends on the nature of the task and the circumstances in which that task is attempted, so ask your GM.