I prefer both UA and AA to DL, with an edge to UA because I'm interested in aawrd redemption USA-Europe and UA's partners offer more and better options than AA's.
Have you had any luck booking biz on LH, Austrian or Swiss using UA miles? Every time I look there is nothing available. What's the trick? I have a lot of miles but not interested in flying steerage to Europe. I find it much easier to book award flights in biz and first on AA or BA using award miles.