My preference is vaguely in favour of a curved neck. Unlike clarinets, sopranos should be held so that the mouthpiece projects more forward than downward. So unless you play with your face pointing at the floor, it is necessary to hold the soprano out from your body when there is a completely straight neck.
I always think the angle should equate with the angle of an alto or tenor which naturally comes out at almost 90 degrees.
For this reason people who need to wear a soprano strap, often end up with a more clarinet like embouchure when playing straight sopranos as the strap would be doing nothing if the sax is held out at the "correct" angle.
I don't like like fully curved sopranos any more, I find them less comfortable, so a curved neck is great for me if I have to play for a long time and might get tired holding a completely straight soprano, but the ultimate choice is going to be for the best sounding option, I'm personally not too bothered about straight or curved, but as I said if I do have a preference it would be for a curved.