Kosher Salt brand? Which brand of K salt? It’s become known that Morton’s and diamond are very different and I’d like to know which to use so that I

The choice is up to you- but I find Diamond Crystal more useful as a finishing salt, than as a cooking salt. In this particular recipe, it's being used to season pasta cooking water, which should be as salty as the sea- or your tears, if you prefer. Salt is otherwise used to taste- and best added in small amounts at a time. In that case, you will learn how "salty" each of your personal size additions is according to which salt you are using. So buy whichever your budget can take, whichever the store has in stock, or whichever you learn to prefer. It really isn't that critical.

Morton's is denser and finer; Diamond is lighter, flakier and easier to to control when adding to a dish.
Given a choice, I would buy Diamond.
If recipe measures are by weight, use either.
If by volume and you want to use one instead of the other, there are articles that give you an exchange factor.