I should say that I'm new to weightlifting, although I've been training with other sports for a number of years.
I started out doing a upper-lower split (at least essentially) with a 5x5 split.
As far as BBB v. a BBing split, I can't speak to the difference from experience. However, I think BBB is as far away conceptually from a BB split as you'll find, a main reason being that a basic principle of BBB is that you should be training a muscle group 3x/week.
I would say first and foremost that the entire philosophies of the two types of program are completely different--with a BBing split, the point is to overload a muscle group on a weekly (or perhaps biweekly) basis and to have that muscle recover fully before being taxed again. The point of BBB is to essentially use the Ramp phases to run your entire body into the ground, after which during the Supergrowth phases your body receives a stimulus response but more rest than it is used to, which allows for supercompensation. A BB split is focused upon stimulation and then full recovery, while BBB is focused upon stimulation to the point where you can't recover, then a supercompensatory period during which your muscles have a chance to breathe.
There are a couple of conceptual points about BBB that are very important to understand:
Frequency is fixed at a minimum of 3x/week because a basic premise of the program is that muscles must be hit within 72 hours or you suffer a detraining effect, and the microperiodization of rep ranges is another foundational premise. You'll be doing 13-15 for two days, 10-12 two days, and 8-10 two days (excepting SG1 and SG3).
As such, the primary variables that BBB manipulates are rest periods, frequency, and volume. Although it's not explicitly stated in the book, the whole point of the three ramps is to find which aspect of training causes the overtraining effect most acutely for your own body. That's why after doing all the Ramps, they say to settle back on a Ramp that worked for you--all the ramps employ different variables, and each person might respond better to one than another.
So, it basically works this way:
Ramp1: manipulates volume and rest periods. This one is, at least IMO, geared to be an introductory ramp.
Ramp2: manipulates frequency and interweek rest periods. You do increase volume, but the primary challenges are hitting the body parts every day and the inter-week rest period shifts, from 150 to 90 to 60.
Ramp3: manipulates frequency and intraweek rest periods. So, your rest periods change as the week progresses.
So, at the end of Ramp1 the transition to SG1 wasn't all that weird. I knew I would be going in with longer rest periods and a lower rep range. However, after Ramp2, it's going to be weird as hell not doing legs every day. I've sort of gotten used to doing so, so it'll be interesting to see how SG goes.