These shields are actually quite different. They both have shadow defense and normal defense, but the line draws there. The status resistances are completely different. But, I think we all know that already.

The difference between the damage defense isn't that huge. Against Fire Fiends, I'd rather have a CoA than the DS shield. Even though DS has more shadow defense, the status is where it really counts. This being said, it is usually agreed that Fire and Shock are much more dangerous status effects than Poison or Freeze, thus making the CoA more valuable in most situations. Poison and Freeze have very simple ways of shaking off, while Shock and Fire will wear down your health and interrupt attacks for quite a while.

As for solo or group play, it really doesn't matter. I think that both shields have the same amount of shield bump, and neither can do much for combat other than that. Relying on shielding or not shielding isn't too much of a question. If a person needs a shield, they will take either one of these. Though, most likely the DS because it is easier to get. A person who doesn't need their shield as often may take Swift Strike instead for its offensive bonus.

As Bamzalot pointed out, status is extremely important on shields. Consequently, the answer is simply this: Against poison and freeze monsters, Dread Skelly is better. Against fire and shock monsters, Crest of Almire is better.

That said, poison is the least important of these four statuses to protect against. In this way, the tie should probably be broken in Crest of Almire's favor. But there are some notable shadow-freeze scenarios, such as Heart of Ice, and holy cow is Dread Skelly useful in those.

They both do different things. It's like asking whether nova driver or umbra driver is better. There isn't a right answer, because you craft them for totally different scenarios.

Dread skelly is probably more useful currently though, but only because of ultimate gloaming wild woods and heart of ice.

Yes, dread skelly, which my computer auto corrects to dread smelly, is good for grasping plateau. I did't think to mention events. I suppose that for that particular event, there are a number of good shields though- omega shell for the harvester, swift strike for going quickly, dread smelly for resisting those stinky grave scarabs.

Except in a shock level?

I believe that a fire max dread smelly shield would still perform worse on fire levels than a similar crest of Almire with no variants....

I like Dread Skelly better, but I don't have one. QwQ.
CoA is better in FSC to be honest...

While a Volcanic plate shield can tank Mace hits better a good FSC team wouldn't put themselves in front of Vanaduke if they have his agro.

COA is good for running the level prior to the boss. VPS is good for tanking Vana or Trojan hits.

I use the Omegaward (basically a Grey Owlite shield with its shock traded for freeze), it can tank a hit from any monster in FSC but it really shines in the Vana Mask Phases where I can hug up real close to the mask and tank several more bullets than COA or VPS.