Back bracing is somewhat simpler to design than top bracing as there is no static loading of consequence. The primary decision is whether to have a live back or not. A live back on a guitar is a shorthand way of saying that the back of the guitar takes quite an active and audible role in the frequency response characteristic of the guitar. The back, if correctly tuned, can add interesting and attractive coloration to the sound of the guitar whilst also limiting some of the deleterious effects of over-coupling between the strings and the guitar top (i.e. the back can be used to control “wolf” notes). However, use of the back in this way requires very precise tuning of the relationship between the back and the top, particularly on nylon strung guitars. Get it wrong and the back will just absorb energy from the string and reduce both the quality and quantity of sound output. For more details, modelling results and practical measurements, please refer to the book.
At the risk of being overly simplistic, a guitar with a well tuned live back will sound more interesting (will have more “tone”) than a guitar with a non-live back, but the guitar with the non-live back will likely have a little more “volume”.
My standard back bracing system is designed so that it can be easily tuned to the correct relationship with the top. Careful design puts the relationship “in the ball park” initially and final tuning can be done after the guitar is strung.
I build practically all my steel string guitars with live backs as I (and the vast majority of my customers) prefer that sound. I build about 80% of my classical guitars with live backs as most of my customers appreciate the additional tonal complexity that the live back delivers over the slight extra volume that the non-live back delivers. However, don’t think you’ll be short of volume with one of my live back guitars. They are more responsive and push out more sound than most other guitars.