Glad to hear there are options now. Last time I was in the market I had to return 5 to finally get a set that even fit with less slop than the worn out trunnions I was replacing.
In answer to your question...flat would give more bearing area, so live longer in theory. Emphasis on "theory"! You must ensure they do not contact the radiused part of the vertical link stud, or that would cause a bind and point of fatigue stresses. Radiused would be an insurance that the trunnion threads would completely clear the vertical link stud radius.
In my thinking, the most important thing about trunnions is that they have minimal slop when threaded onto the vertical links. You should just barely be able to feel a motion between the 2. If there is enough to make an audible "click" as you rock them together, then that is too much. Too much play gives the forces a "running start" to hammer the link every time you hit the brakes.