I sent Delta an email:
From: Jason C
Sent: Tuesday, March 08, 2016 8:15 PM
To: Customer Care <email@example.com>
Subject: 50-767 dust collection bag mounts
The 50-767 offers both a cloth-covered ring and a metal clamp
as options for attaching the collection bag.
What is the difference between these two options? Does one provide a
better seal? Is it a tradeoff of convenience vs. seal? Why would you
choose one over the other?
And they said:
It’s just a personal preference. You get a good seal w/either. Our tech dept can be reached at 866-999-1499 if you have further questions.
So it doesn't seem to matter.
Now, if you look at the manual, the last step for the ring option is:
- Tug slightly on the bag to eliminate any slack and to make sure that the bag is secure and will not detach during collection.
This does seem to imply that there is at least enough of a risk of the user attaching the bag incorrectly with this method to mention it in the manual. This seems to be user error, though, not attachment failure. It also seems to be a minor risk, based on the customer service reply and the fact that nobody really seems to have a strong opinion either way (lack of answers posted to this question is some example evidence here).
I hypothesize (this is just my personal opinion), that the reason both methods were provided is that both are good, and it is cheaper and easier for Delta to provide both methods and let the user choose than it is for them to potentially drive away customers that have a preference one way or the other (plus, empowering the user to make a decision, even a small one, is a nice way to improve the product image). I can certainly imagine that some folks may e.g. trust the clamp over the ring, and others may value the convenience of the ring - and regardless of whether or not either of these preferences is based in truth, by providing both, Delta doesn't have to deal with a debate.
(Or it may be an even simpler reason: Maybe in a design meeting they got into a heated debate about which one to use, until one sane hero stepped up and said "let's just use both; next discussion item please, I don't have all day".)