You will always hear different things about order, because everybody has an opinion, everybody has different setups, different AIO's, different fan settings etc.
When dealing with rads, static pressure is important, just as much as cfm. It's the relationship of both that sets up the performance variables. In an engine it'd be equivalent to torque and horsepower. There's also a relationship between the rad/coolant temps and ambient temps. In traditional top mount, the air used is case ambient, which is generally 6-12°C higher than outside ambient, so affects the temp of the coolant and its ability to absorb cpu energy.
A small amount, you'd generally see cpu temps 1-3°C lower with front mount rads.
That doesn't take into consideration the fan rpm or orientation.
In push, the majority of air is forced through the fins in a donut shaped area, you'll see that easily with dust buildup, because a fan is round the cone is round, and nothing gets pushed from the motor housing. In pull, the vacuum created by the blades has no real shape, it affects just about the entire area of the rad surface, greater area = greater capacity to cool.
This changes with rpm. Above @ 1200-1500rpm there's generally enough cfm that ability is flipped, there's more air directed across the fins in a higher concentration. In pull, the vacuum is weaker relatively than the exhaust. This can change cpu temps roughly 1-3°C.
So for low rpm, front mount pull can see @ 4-5°C total lower cpu temps, for high rpm, front mount push.
But that's just cpu. The exhaust from the rad will now change case ambient temps slightly, which in turn can affect the gpu by 1-3°C, as it too uses case ambient temps for cooling. But thats a different argument as to why ppl will mount an aio as exhaust.
Overall pc temps don't really change much at all, there's bonuses and detriment to any orientation, rpm, mount, it's going to boil down to your exact preference, what you can/cannot live with, what looks best, what fits. When cpu temps are well under @ 70ish°C, even 5°C doesn't make a hill of beans difference one way or the other.
Except to your opinion.