I use a Dura Ace cassette because (1) I can't use the Ultegra 28T cassette with Di2, and (2) they don't make an Ultegra 27T cassette. As long lasting as the DA cassette is, I'm amortizing the extra cost over thousands of miles. So far, using DA (and getting the gearing I want) has cost me 2.5 cents per mile over an Ultegra cassette. By the time my DA cassette is done, my cost per mile will likely be half that. Not a bad deal.