One point of confusion you may have come across. Some people interpret “split” to mean “no compound movements, all isolation movements”, which is an incorrect way of looking at it. From an aesthetics standpoint, I think you’ll find that most people agree that both compound movements and isolation movements are necessary, even on a bodybuilding split. While there’s some debate whether to do the compounds first or last, nobody’s actually saying “do only isolation movements”.
That out of the way, I’d look at it this way. You only have so much energy to put into any given workout. So on a day that you’re training your shoulders (whether it be a dedicated shoulder day, an upper body day, or whatever), your shoulders can get more stimulation from a split than they can with a full-body workout, just on the basis of having more energy to focus on them.
(This is exactly what triceptaurus said above.)
There’s also the issue of frequency. With most full-body routines, you’ll be working out the same body part 2-3x a week. With many splits, you’ll only hit that body part 1x a week; it’ll receive more stimulation at a given time, but only once over the course of the week. However, with an upper/lower split, you can potentially hit that body part 3x a week too, if you’re willing to be in the gym 6 days a week.
So, as other posters hit on, the other factor is really lifestyle. How many days are you willing to lift? How often are you going to end up skipping a day or two?
In the end, it mostly comes down to manipulating energy, recovery, training stimulus, and frequency. Full-body routines and splits just approach that differently. You can get great results with either, but in my experience, a simple split gives you a bit more flexibility to make adjustments.
My personal split is:
And I just keep rotating through it; that gives me a minimum of 72hrs recovery time. Sometimes I’ll skip a day or two and just pick up where I left off. If I want to try a specific training protocol, I can phase that in and out without messing up anything else. Say, for instance, that I want to try an 8x8 routine for my chest, or try out a John Meadow’s routine for my shoulders. I can do that without changing my split, my schedule, or the way I train any other part of my body.