We tried the Cozy Kitchen recipe and Mr wtg has pronounced it a winner and even better than his (former) favorite from Panera. I ended up making half a recipe.
I think the Smitten Kitchen recipe is actually quite similar in terms of proportions. Maybe a tad less butter and flour and a little more liquid, which might make it less thick. Though I have to say the CK version was not absurdly thick.
I would definitely stick with fresh broccoli. You can really taste the difference.
This will be an ongoing experiment in the wtg house. Not sure whether to try the SK recipe first, or to stick with CK and change up the cheese. This time around we used a Cabot cheddar that was a limited edition/special reserve/whatever. Excellent cheese all around, but not something that's stocked all the time at Costco.
Or we could use the same cheese and try the SK recipe. I do like the little additions of the seasonings in the CK recipe: mustard powder, hot sauce, Worcestershire sauce, and nutmeg. You don't really taste any of those ingredients but I think they add "something" to the overall flavor.