I own a Turbonetics Deltagate-MarkII (Racegate). Although I have never personally used it in a twin turbo diesel setup (long story... :{ ), I've worked on a few vehicles over the years that had 'em and have had very good luck with them. I've also worked on a few vehicles with HKS wastegates... they weren't too bad.
There are many folks running the 42mm TiAl with pretty good luck. The best wastegate is probably the Turbonetics NewGen... but they're insane money.
Matt