IMO, tone bars, and all top bracing, are mostly there for structural reasons, and have only a 'fine tuning' effect on tone.
It's interesting that if you look at the vibration modes of a top that has no bracing, they're quite similar in shape to the modes that seem to work well on the assembled guitar, but lower in pitch. In theory you could make the top thick enough to move the modes up to where they're 'supposed' to be, and it would probably be stiff enough to hold up under string tension. It would also be pretty darned heavy, and probably would not have much volume, although the sustain might be terrific. In that sense, then, what we're doing with the bracing is trying to make the top stiff enough to hold up under tension and keep the weight down so that it will respond better.
As Mark Blanchard pointed out in his H'burg talk last summer, and as I can confirm, it's difficult to make the top resonant modes come out 'right' simply by changing the bracing if the stiffness characteristics of the top aren't 'right' to begin with. It takes a fairly large change in brace angle to make up for a top that is somewhat floppy in the cross grain direction, which is why we spend so much time picking tops. Carleen Hutchins used to say that plate tuning (or tap tones) works best when it 'just happens': the more heroic effort you have to put in to make things come out right the less likely you are to end up with a great instrument.
This is far from saying that these 'fine tuning' effect are pointless. As in any field that is dominated by good designs the differences between the 'best' and the 'average' examples are small indeed; often too small to measure. In a case like that any tiny advantage is likely to pay off big, and it well worth pursuing.
So: low and wide, or tall and narrow? Whatever works for you.
There will be differences for sure, but since so much of what constitutes 'good' in this business is subjective, you'll have to decide whether one way or the other is your prefered method.
Objectively, I'll note that there is a limit to how tall and narrow you can make a brace, set, I think, by the strength of the glue line, if nothing else. I'll note that lute top braces tend to be about five times as tall as they are wide; I suspect that's a practical limit.
Tall narrow braces should weigh less for a given stiffness, in theory. In practice, since most of the mass of the top is in the top itself, rather than the bracing, it might not matter too much. If the bracing is 30% of the total mass of the top, and you can shave the brace weight by 10%, you've saved 3% of the total mass of the top. Is that important? You could have saved that much by picking a slightly better piece of wood, or sanding it a little thinner. If you've never had one fold up on you, you're probably making them too heavy anyway.
A tall, narrow brace will be much stiffer than the area around it: in physical terms, there's a large 'impedance mismatch'. That spot will tend to be 'nailed down', since the rest of the top won't be able to convince it to move easily. Tall, narrow braces tend to break up the vibrating areas of the top at high frequencies.
In the end it always seems to work out that 'moderation in all things' is a good guiding principle. If you start out with a 'standard' design, and learn to build that well, you'll probably end up with a decent instrument. If it's not exactly what you wanted (and it never is) then you can think about 'tweaks' that might move it more in the desired direction on the next one.
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