Stop confusing yourselves ^^
If by diameter you mean circumference/pi and nothing else, then it ONLY applies to perfect circles/cylinders(/balls :D ) and you can forget about it here.
If however, you simply mean the distance from one side of an object to the other side from a certain point of view, then diameter=width.
Regarding the actual question here: Yes, it could make a difference if you gain girth by gaining width or by gaining depth/height.
Increasing the dimension that is already larger (mostly width I should guess) might stretch a vagina more noticeably and allowing for more friction.
Increasing the smaller dimension increases girth faster but stretches a vagina differently.
And no, I have no I Idea which one is better. Both should be beneficial and we can’t really choose one so I guess the question really is moot.