No idea as I've not tasted weber's.... for comparison:
Weber's Horseradish Mustard:
Ingredients: mustard seed, vinegar, water, horseradish, tumeric, red pepper, salt, natural flavorings, guar gum, xanthan gum, ascorbic acid, soybean oil, sodium benzoate (a preservative), red #40
Kelly's Mustard:
Ingredients: mustard, sugar, vinegar, horseradish, mustard flour, lemon juice, mustard seed, salt, garlic, spices, xanthan gum.
The kelly's is definitely more of a brown/deli style mustard than a traditional yellow/prepared style mustard. Not sure which category Weber's falls into/