Rending Cleave is better for wave clear since Psionic Strength requires hitting heroes to proc. 40% more wave damage at level 4 is good since not only does it make his wave clear even better, it does so when wave clear is still meaningful and by end game it out right one shots minion waves at a lower level than without the talent.
Rending Cleave also works fine without Might of the Nerazim, such as if you are taking Void Prison to combo with allies. The 3 second duration stacks with the 3 second cooldown on cleave so there is no wasted DoT, and building up stacks is slower due to the no skill reset.
The DoT also has value with Shadow Mending since it provides healing over 3 seconds during which time you might take damage rather than burst healing that might be wasted if you are already close to 100% health.
Cleave build is typically paired with Void Prison rather than MotN. So it provides better wave clear and relatively simpler mechanics, along with insane CC in VP.