Are dry or fresh whelks the better choice for whelk soup (響螺湯)?

"should I use fresh or dried whelks", well you can use whatever you want. But there will be a difference. Why is this even a question? Let's address a couple points.

Why is dried seafood used (Whelks, abalone, scallops, etc.)?:

• Storage and transportation: In the days before refrigeration, refrigerated trucks / trains were available (and still in areas where they are not common) the only way to make these seafoods last long enough to even reach the consumer was to dry them. Drying these foods is a means of preservation, primarily.
• Taste & Texture: Drying seafood intensifies the flavor (in some cases) and alters the texture. These two characteristics have been deemed desirable in some dishes. The argument should be made that this judgement is subjective. Some people may prefer the flavor & texture of fresh over dried. The majority of Chinese & Japanese cooking I am familiar with would clearly put an emphasis on dried. That said, there are many recipes out there that allow for the use of either. Linked here are several abalone recipes, some of which instruct on substituting one for the other (sorry, I couldn't find such a list of recipes for whelks. Abalone is treated similarly in Chinese cooking to whelk, they are both sea snails).

So, if you have both fresh & dried whelks available to you, try both & decide which you like better.