You've got them, right there. I've never used DAG, but he's acknowledged by those who have as possibly the best there is. I *have* used Youxin Ye for repair on my M3, and his work was very good, too--I was entirely happy and would absolutely use him again.
DAG will probably cost a bit more and have a longer turnaround time, but from what I understand he's capable of doing things with lenses that go a bit beyond what Youxin offers. Youxin does have the advantage that, if you pay extra (30%? I think that's what it costs) you can get expedited service. As his turnaround times are creeping upward, if you need the lens back quickly, he's probably the way to go.
My understanding is that Youxin is much more communicative than DAG. His communication before I sent in the camera was excellent, and he got in touch quickly with me when he discovered that someone who'd previously worked on the camera had somewhat buggered something inside, and he fixed it, too, without extra charge. Obviously, you don't want to go bugging your repair person--it can be annoying to them, busy as they are, and answering calls/replying to e-mails takes time that could be spent working on cameras--but if you *do* need to get in touch with them, it's comforting knowing that you can. And from what I've heard (and experienced), Youxin scores somewhat higher on that front.
Really, with either you can't lose. Because they are both in high demand, be prepared for a long wait to get your lens back unless you go with Youxin and pay extra. Neither is slow, they're just swamped, really, with work. It's great to have both still doing such excellent service.
