Now with added screenshots for extra clarity! My apologies for not providing them immediately! >n<

First of all, this post is not about who is stronger between the two or anything like that. They have similar roles, Noel has higher mitigation but lower stats, those are old news.

But being lucky enough to have both those units, I was able to check the properties of their mitigating provokes in battle, and, interestingly enough, they are not buffs of the same kind, which might allow some peculiar interactions between damage reduction effects.

After using Impregnable or Guard Smite, Wilhelm receives the "Reduces damage" buff, which mitigates all forms of damage, magical and physical alike. It's the same buff given by Sakura's Defensive Barrier, Y'shtola's limit and Wilhelm's own limit. Being identical buffs, only the most powerful one will apply at any given time.

Noel's Bring It On and Steel, however, act differently: each skill activates​ two separate buffs at once, named "Reduces physical damage" and "Boosts magic damage resistance"; the first one is identical to the one provided by Golem's summon, while the second one seems to be unique to Noel alone, at least for now.

While this is a seemingly minor detail, it means that Noel cannot take advantage of Golem's summon to further boost his physical defense, but his "double" mitigation should stack with other damage reduction effects (don't know to which degree, though), making him even more indestructible than he already is. Conversely, Wilhelm can become nearly immune to physical damage under Golem's buffs, but won't take advantage from other mitigation buffs (unless they're stronger than his own, like maxed limits are).

I don't know if this little piece of information will be useful to anyone, but it seemed quite unique and interesting and I thought I'd share it with you all. Maybe someone will make some wicked strong strategies out of this, who knows :3

EDIT: /u/abudangdut provided a picture of the mitigations stacking on Noel! Hope this makes things more clear now!

EDIT2: I made several screenshots to show all the cases I talked about, but I won't be able to upload them for a few hours. I'll update this post with them as soon as I have the chance! :D

EDIT3: And here they are! I represented all relevant cases and interactions between the skills, hope this helps to make everything more understandable:

1) Wilhelm using Impregnable alone

2) Noel using Bring It On alone

3) Wilhelm using Guard Smite alone

4) Noel using Steel alone

5) Wilhelm under Impregnable Y'shtola's Aetherial Pulse

6) Noel under Bring It On Y'shtola's Aetherial Pulse

7) Wilhelm under Impregnable Golem's Earthen Wall

8) Noel under Bring It On Golem's Earthen Wall

As you can see, the two buffs work and interact with other mitigations differently. Again, my apologies for not adding screenshots when I posted this. Hope this makes up for it.

TL;DR - Wilhelm's mitigation stacks only with Golem's one; Noel's mitigation stacks with all other mitigation skills except for Golem's one, thus providing him with even more defense and tankiness.