It's a matter of personal preference, but I'll give you the reasons I get insurance from a third party rather from
DCL:
I've always read that it's better to have the insurance from a 'disinterested party' rather than from the travel provider in the case of a dispute. The reason would be that the third party wouldn't be interested in settling claims in favor of DCL and would be more likely to look at it objectively.
I find it more convenient to buy from a 3rd party, but I carry the paperwork with me so that it's easy to identify that I do have it, in the event I would need a medical evacuation or other emergency.
I can comparison shop and find a provider that has a good price with the major features that I want.
I can wait and buy it as close as 2 or 3 days before the cruise, if I wanted.
If I buy it within the first 7 days of booking, I get a waiver that covers pre-existing conditions.
Some of the pros of buying the DCL insurance are:
One stop shopping with one payment.
Easy identification if you need the medical evacuation - don't need any additional verification or proof of insurance because it's on the DCL records.
hope that helps