As is the case with so many workout questions, this one comes down mostly to context. Which format you choose depends on both your training goals and your training frequency.
I tend to use full body workouts in two main circumstances. First, and probably the most common, is with body composition workouts. Not every body composition workout, of course, but a good chunk of them. German Body Composition workouts, for example, are centered on full-body workouts because of the specific training effect that comes from using both upper and lower body exercises in quick succession. By alternating between compound upper and lower body exercises (back squat and upright row, for example), you force your body to shuttle resources between opposite ends of your body. This means that your heart and lungs have to work especially hard, which is why this format is notoriously exhausting and great for fat loss.
Full body workouts also make good sense if your training schedule is limited or inconsistent. If you’re only able to workout two days per week, then splitting these sessions into upper and lower body workouts leaves too much space between exposures of each body part. By doing a little-bit-of-everything with each day, you get to tax each area with enough frequency to drive some progress.
Body part splits work well for athletic development, hypertrophy, (building muscle), and general strength and wellness. In each of these settings (especially hypertrophy) there’s value in accumulating lots of volume on one particular body part or motor pattern. If you know that you need to pile up 1400 seconds of time under tension with your quads (yes, we actually count seconds worth of movement in your P/S workouts), narrowing your focus to one body part becomes imperative. Otherwise you simply won’t have enough time in your session.
Similarly, when you accumulate enough volume on one body part, you’ll need time to rest between exposures. So after a nasty leg day you get 3 or 4 days before you need to tackle those same muscles again. So body part splits allow for both adequate volume during each training session and adequate recovery between each session.
Happy lifting –