They aren't really comparable in that sense.
Artificer armour really is only power armour, with those strengths and weaknesses, that has been made extra fine by additional work that gamewise is easiest to represent by increasing the save, does this mean that can you expect a set of power armour to burst open when hit by a missile while an artificer armour just shrugs it off? No. In reality an artificer armour is simply very wellmade, making use of rare or expensive technologies that it might be impossible or simply not worth it to fit into your average Space Marine's armour that makes the suit work more efficient and withstand some damage better, it might be built specifically for the officer using the armour with special modification to make the suit more to his liking but in the end it is just power armour.
It's like comparing a car to a custom-made one, both are cars, both drive on the road but one will has been made with more care and consideration, making it more enjoyable for the driver and less likely to break down. In this comparison the terminator armour is a tank.
As for first captains, while authority equals asskicking in 40k you must still hold that authority first. A first captain is not a champion of the legion (there are those too) but an officer so for him to have risen to the position of first captain he has to have been a damn good one I would assume. If this means that someone who doesn't take particularly much to using terminator armour finds himself in that position he will most likely forgo that and fight the way he is best at fighting. It is also worth noting that terminator armour isn't the most suitable armour in all circumstances since it is quite unwieldy compared to power armour. Also, not all legions might necessarily find the employment of terminator armour as something specific to the company belonging to the first captain, many legions seem to have their terminator retinue as a specific bodyguard to the Primarch, in some cases this will mean that the first captain is the leader of that bodyguard (Iron Hands comes to mind) but that doesn't have to be the case for all of them.