I've just switched from a neck strap to a harness and can say that for me, it has made a large difference in a variety of areas. In terms of sound - yes, I'd say my tone is somewhat better with the harness, but I think this is a natural by product of the better posture that the harness affords, rather than anything special about the harness itself. It does take the pressure of my neck, and it allows me to have much better standing posture (I don't sit much) = better / more efficient delivery of the air = bigger / better tone. We are talking about small improvements here, a harness is not a magic bullit for tone problems.
That said, I don't know if I would suggest running out and switching any of your gear before an important event unless something is very wrong. Play the exam etc with what you're are comfortable with, and then switch.
In terms of brand ... just go and try some on, you'll quickly figure out what feels good and what doesn't.
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