The T21 is still being made by Rolex, so their dealer/service network pricing will be lower than the price we pay on ebay. Your watchmaker will have to tell you how much lower since I do not have a parts account.
The stock (aftermarket) crystal that comes with Daytona reps (including DWs) are junk. They are also made of lighter gage (thinner) plastic than the gens. I have never used Clark's T21, but all of the other crystals I have bought from them were constructed similar to the gen Tropics.
Noticeable is the key-word. Whether the sparkle, beveling & higher degree of transparency of a gen T21 is noticeable to you is the question. But, in general, the gen Tropics tend to give a watch a jewel-like quality that most aftermarket crystals (including Ofrei's) cannot match. There have been a number of picture comparison threads over the years (1 or 2 by me, but I think Nanuq's are the most revealing) that you might be able to locate via the Search.
The best aftermarkets I have found are from Clarks.
Functionally, they should be the same, but I believe the screw portions are different. That means you can install either pusher into either Daytona, but the screw may look different. However, to be sure, you might PM Ubi (unless he reads this thread & chimes in 1st) since he knows more about 16520 components than I do.