I own 4 DW revolvers, all Monsons. I haven't gotten the chance to fire the 2 x .357s much yet, but I have put quite a few rounds through the 2 x .44s, and they are superb shooters, and quite attractive, as well.
I wouldn't want to say that one is definitely superior to the other. Certainly the ability to swap barrels is useful, and makes for easier cleaning. One could also argue that DW has closed their doors multiple times, while S&W has soldiered on, and that might say something about quality.
I like both my Smiths and DWs. Any would likely serve their purposes well. Buy whichever appeals to you more, and/or whichever one best fills your needs/wants.
Tim