Depends on certain things. Looks, serviceability and usage.
Looks is up to you. Some prefer to see the fans because of beauty rings, leds, colors etc, some don't or don't care.
Serviceability. Typically, dual radiators ate a pain to clean right. Mounted on top of 2x fans under a case lid isn't the easiest thing to deal with. Mounting the fans to the case then the rad to the fans means not having to remove the fans, just hit the radiator surface with a soft bristle paint brush and a vacuum.
Usage. If the fans are normally seeing high usage under @1500rpm, pull is typically more effective than push. If fans generally see higher than @1500rpm then push is more effective. It's not a large difference, usually 2-3° at most, but some are...