When the radiator sits in the top, and works as an exhaust,
with no dust filter like in this case.
I would probably likely put the fans ontop of the radiator.
Why? because i think it would be easier to clean dust.
When you put the fans on the bottom the rad,
there will always be clogged dust between the fans and the actual radiator.
So if you need to clean that out, you will have to take off the fans.
When you put the fans ontop of the rad as being an exhaust.
Then there will be more dust collection on the bottom of the rad.
But that will be way easier to clean then.
Because there are no fans in the way.
Of course there will always get some dust trough the radiator itself.
But still i suppose it will be way less cloggy with the fans ontop,
in comparison with the fans on the bottom.
Of course looking at it from a different perspective.
If you want to clean the dust out ontop of the rad.
You will have to demontage the entire rad or top half of the case when possible.
So yeah eitherway has it’s pro´s and cons.
When you have decent dust filtering inside the case,
then it will likely not matter really much.
So yeah either way works.
Of course putting fans on the top pushing air out trough the radiator instead of sucking,
might lead to a slightly better airflow.
But again that is also strongly depending on your intake situation.