If you take Massalia early on (it's tempting because of the big port, the strategic position, and the excellent spearmen roster that you won't get anywhere else in Gaul), Rome will throw endless stacks of heavy armour against you. Which
is scary, because you can't recruit AP units in Massalia, apart from slingers. Meaning you'll probably have to periodically reinforce the town with axemen from other provinces.
But then Rome won't expand. What's the point of facing a Eoman army that can't get out of Italy?
Not exactly, but the point is they are similar to your own units, recruitable in the same barracks type, in region where you can't recruit any of your own units.
Celto-Germanic Spearmen are very helpful if you conquer Gaul, because Swęboz can't get Lugoae (not the greatest of units, anyway) or any other spearmen in Gaul itself. They also have a good shield and long range javelins, which is rare among Germanic units. Also useful for Gauls because Gauls can't get Dugunthiz.
Celto-Hellenic Hoplites are armoured and thus fill a vacant niche in the Gallic roster (medium spearmen). They are a lot like Thureophoroi. Very useful unit. Arjos, meanwhile, are heavy spearmen (elite or semi-elite) and relatively expensive, and you can only recruit them in one region.
Note the huge AoR on both these units. This is what sets them apart from other useful spearmen types in the region (Belgae, Massilian Hoplites, or any of the Alpine units). Gaelaiche are good too, but they don't have armour.
I like the massalian hoplites among them all, as well as the Solduros for their stats. Better than Cordinau orca.
I don't know, you'll have to figure out for yourself I guess.
That the problem.
I like the Aedui skins better (except for the general). But I guess it's a matter of taste.
But the Solduros have round shields as Aedui, with red color. The Solduros of the Arverni have hexagonal shields with black and yellow. Plus the Arverni neitos have cool capes. Although ,they do have visibly smaller shields.