Theoretically, balanced design is quieter, though this does not mean it sounds better. Just as a more efficient loudspeaker does not mean it will sound better than a less efficient loudspeaker. The true balanced design will cost a lot more money, because you will have 4 seperate signal paths to follow opposed to the standard two paths of an unbalanced design. More paths = more parts = more costs.
So some "balanced" designs will not sound as good because they will use less expensive parts to compete finacially with the unbalanced designs. For example, using the same design and parts, and taking dealer mark up into account, a $7.5K truely balanced preamp will sound the same sonically as a $5K unbalanced preamp (in a area with low EMI/RFI issues). The total price wouldn't be double because they can used the same chassis and a similar power supply. OTOH, if both preamps are priced the same, the unbalanced design will have the advantage of being able to use better parts and should sound better, again, unless you live in a higher EMI/RFI area.
Now if you live in an area where their is high EMI, RFI or you have grounding problems, the extra cost for balance may well be worth it. If you live in a place where EMI, RFI and ground issues are null, than your money is probably better spent on an unbalanced design.
I've gone both routes, and neither one is right or wrong. I've found that in my system, fully balanced is slightly quieter, barely noticeable. I would say that I notice more noise levels between tube gear and SS gear than between balanced vs. unbalanced (I still prefer tubes). That said, noise is not really much of an issue in my system, and I'm currently using unbalanced gear. YMMV.
Cheers,
John
