What is your objective?
Do you have a budget?
Do you now have a i7-8700K?
How are temperatures now?
There are several motivations to buy a liquid cooler.
Some just want to do something different. I might guess that is you.
That is ok and a good reason.
With a good cooling case like either ethoo case you can mount a top end air cooler that will cool as well as a 240 aio cooler. Something like a noctua NH=D15 will be more reliable and quieter.
No risk of leaking.
A 360 aio cooler will give you the top cooling available, if you are planning on max overclocks. But, you say you are not interested in that.
I like an aio in a space constricted case, but your cases have plenty of room for any air tower you want.
Mounting a radiator is a bit of catch 22.
If you mount it in front to take in fresh air, you will get optimum cooling for your cpu.
You can also filter that front intake to keep your parts cleaner.
But, your motherboard and graphics card will get the hot exhaust to use for cooling.
OTOH, if you mount it on top to exhaust air, your cpu will be using warmer air for cooling.
You may also have some extra cash that you are itching to spend.
Go ahead and scratch that itch.
If you do not have a ssd, that would be my first suggestion.
Or, perhaps a second or better monitor.