I watch both and will continue to watch both (DFTL only because I don't have anything better to watch on Weds at 9:30.). From the beginning, I was under the obligation to like TSB better because I know one of the 'professional' singers in it (they make a really good chunk of change, by the way). But, lo and behold, I actually liked it better. IMO, DFTL takes too long to get anywhere productive. In TSB, you know that you're going to have a winner by the end of the show.
I also like it better because I think it's harder because in DFTL, you just sing the whole song. In that case, the words generally just spit out where as in TSB, you have to think about it. Well, it makes sense in my head and I can't really get it to make sense here but that's okay.