After trying several different things, it does seem hitting everything twice is superior. That also makes sense from a mechanistic perspective, as hitting 30 sets over the course of 2 days, as opposed to 1, enhances the propensity toward a myogenic gene expression modality and leaves protein synthesis elevated longer. The only caveat here is that it's likely dependent on training history.. ie, a complete beginner could easily get away with more and so on.
Ergo, a push/pull/legs/push/pull/legs/off/repeat is my ideal split now. It's not that I'm opposed to something different, it's just hard to come up with a different split that allows hitting each body part twice a week without severe overlap and still enables high volume, which is unequivocally optimal for hypertrophy.
To break it down some, high volume is concomitantly the most and least important aspects, paradoxically. You must lift with a certain % of your max (intensity) to fully stimulate and activate all motor units/muscle fibers adequately. The recent study from Brad corroborated that. Furthermore, for the reasons I briefly outlined, hitting everything twice is undoubtedly optimal, and at this moment, there's no reason to think otherwise. Thus, increasing volume is the only real component you can play around with and increase (similar to CHOs in your diet, for instance.. protein/fats should be RELATIVELY stable based on a necessity).
From Brad's book (he's basically the most prominent figure in the field of exploring hypertrophy.. I hate referencing him so much, but he's done by far more than anyone else) and from strictly mechanistic insight again, keeping a relatively controlled/slow negative is ideal for hyp as well (ie 2-3 secs.. I usually stick with 2 secs as a good standard). A forceful concentric portion is fine/ideal as well, just ensure you actually squeeze the weight somewhat (don't go overboard, that's also a mistake). Mind-muscle connection is actually quite integral to true progression, as to ensure optimal contraction.. we are going for hypertrophy after all and don't want to just "move" the weight with literally no care/bearing for our actual muscular activation. Rest periods, anywhere from 45 secs-2 mins is probably ideal. The studies on this subject are more scarce, however, it's definitive that lower rest times will stimulate greater metabolic stress which likely translates to augmented hyp. Optimal rep range is around 10-15 for fast twitch predominant fibers and 15-20 for slow twitch predominant fibers.
Purpose of the higher time-under-tension (based on 2 sec negatives and high rep sets) and low rest times is to augment metabolic stress, resulting in maximal muscular micro-tears necessitating celeritous repair. Reaching lactic acid threshold (plus other metabolites) as well will cause an increase in extracellular (surrounding the muscle) acidity (lower pH) causing further fiber degredation, membrane rupturing, and general membrane perturbations (apart from the intrinsic aspects such as cross-bridge cycling, mechanotransduction, etc) to fully maximize ALL possible anabolic pathways ie; mTOR-p70s6k pathways, MAPK pathways and Ca2+ - calcineurin pathways. Not only that, this type of training orienting itself in general will cause further hypertrophy of different non-contractile proteins (ie; desmin, dystrophin, alpha-actinin, nebulin, titin, et cetera) and cytoskeletal rearrangement (ie sarcoplasmic gain), as well as increased cell swelling/water & glycogen retention. With that said, higher volume routines also stimulate greater bouts of protein synthesis (though I'm not one to typically look at the fallacy of ONLY measuring acute protein synthesis rates, it certainly plays a role).
Furthermore, following w/ the previous section: there's good evidence to suggest that higher volume ALSO stimulates greater satellite cell proliferation (and ofc MyoD upregulation, coinciding w/ the proliferation). Most types of training will equate to robust satellite cell differentiation and induction, though higher volume, typical hyp training even equates to greater myonuclei donation (via greater myogenin increases) than do other types of training. The difference in proliferation is more profound but both are noteworthy. Much of that can likely be attributed to the muscular shearing and such causing a more robust immune cascade following training.
So yeah, my favorite hyp workout is outlined above.. and the reasons are also given why it probably elicits the greatest hyp adaptation as well.
With that said, my favorite form of progression is increasing sets every so often.. it's seemed to work the best for me. Volume = weight x reps x sets. Your reps will generally remain stable (10-15) whilst your weight slowly creeps up. Esp as you continue progressive overload and progression and get into some decent weight, a hyp workout is going to provide quite slow weight progression. For that reason I like to slowly add sets, say, bi-weekly over the course of a long period of time. This gives me another form of progressive overload irrespective of any governed by the canonical weight stack increases.
I try to aim for roughly 30 sets per large body part per week (around 16-20, direct for bis and tris, separately). One thing I would do also to acclimate to the higher volume is the employ the progression I outlined.. ie start the bulk modest (say, 24 sets/week for that bp), then slowly creep up the sets over a few months until you're at 30+. When you begin feeling run down via the overreaching that's probably occuring, deload and get back at it.