Using the aigis.gcwiki.info I have manged to make some rough calculations on the value and comparison for Maribel (forgive my spelling)

Now when affection and awakening are not taken into account and we only focus on max level cc, we find that Maribel only has
2329 health, 178 attack, 307 defense

Doing the same with Leanne (she is our base silver for comparison), we have
1970 health, 213 attack, 426 defense

Maribel we see excels in attack but lacks in health and defense which are the most important qualities of heavy armors. Being the tanks of aigis we (or at least me) are more concerned with them taking hits rather than dealing them.

Now we will take skill into account (skills will be assumed to be maxed)

Maribel's doubles both attack and defense so we now have
356 attack, 614 defense

Leanne's skill increases only defense by 60% so we have
213 attack, 681 defense

Again we see that Maribel focus's on attack but Leanne excels in defense which is the main goal of a heavy armor

Now we shall see when affection is taken into account (for the sake of space I will condense the details into one line)

Maribel now has 298 attack, 367 defense - with skill we have 596 attack, 734 defense

Leanne now has 558 defense (attack does not get boosted) - with skill we have 892 defense

As we can see, even without awakening, Leanne is on par with a max level awakened black unit in terms of defense whereas Maribel just barely scratches the surface. In order for Maribel to excel Leanne she would need to be awakened as well and max level which gives us
(with affection and skill)
648 attack, 972 defense

Now Maribel is on par with the max level awakened black units, but is it worth it? In the long run of course but considering the difficulty in maxing her level, skill, affection, and awakening,(and just getting her) it will be a long ways off. For the sheer sake of ease, Leanne is still a better unit to focus on in the short term as she is far easier to max in all areas.