I have schemes for both Machine-based and Chaos-based upgrades, as well as Attack-based upgrades.
We need to understand the purpose of the mod.
Chaos-based upgrades give an advantage to those with a high chance of Machine and Chaos creatures appearing (demons).
Attack-based upgrades are the standard option for most players.
Machine-based upgrades are for single-player (enhancement) or multiplayer (alternative).
But in general, in my opinion, 3 > 2 > 1. Machine operators are usually warriors (too many perks are spent on ballista, leaving not enough to fully develop magic), therefore, firearms in the attack skill will at least be accessible to everyone, giving all warriors the opportunity to acquire high-quality machines. In addition, in the original game, firearms usually go through the ranged attack skill, which is rarely taken anyway (due to the unconvincing perk compared to Battle Fury, Mass Haste, and Retribution), which means that firearms clutter the build less than in the Chaos skill.
By the way, there is a way to get rid of the "clutter" (both in Chaos and Attack), by linking the upgrade to the Rapid Shot skill (but these are external links, which add more complexity).