I've flown both fairly extensively. If comfort of the seat is most important, then the skybed is the best. But you cant get that, so I'd say take the UA C seat. It has more recline, and there is more pitch in the UA seats than the dreamtime seats it seems. I haven't seen the measurements though.

There are other considerations too. The service is usually better on QF, and I say usually, since QF cabin crew are notoriously inconsistent. Food and Wine is better on QF as well.

There is a third choice too, Air NZ, connecting in AKL. They are refitting their planes with a virgin style upper class suite in business, but that hasn't happened yet. This leaves the NZ seats a distant 3rd. Of course, the service on NZ is miles ahead of QF and UA.

Personally, I'd go UA since the seats are the most important thing to me--I'll be sleeping for 10 hours hopefully. (That and the MP program is miles ahead of the QFF program, see my comparison.)