5-8 reps typically builds real muscle while 12 and upwards seems to build better glycogen storage capacity. For body builders it’s best to get both. Increasing glycogen in your muscles usually means you can lift a bit more weight (increases contractile ability).
Glycogen capacity can be noticed pretty quick in size gains, which is why if you ever hear someone say they got massive gains in 2-3 weeks (without juice) it’s all water and glycogen! It takes a while to notice what is called “dry gains” as it happens much more slowly, this is actual protein growth (increased muscle cell size and new cells). Glycogen on the other hand brings 3 grams of water with it which is why it’s a lot more noticeable size wise if you increase the storage capacity of it.
So yeah, mix it up, do low (about 6) rep sets, and top it off with a pump inducing, slightly less weight, high rep set (I go for 12-15). Or just mix routines up every few weeks like most people do. Only doing ONE or the other won’t give you the best SIZE or STRENGTH results.