I think the "heat rises" argument has been debunked, the air in PC cases moves where the fans push it, it doesn't have time for the heat to cause it to rise.
But the top fans are traditionally used as exhaust either way.
I'm currently using a 240mm AIO, but one row of small fins has come loose and it's being RMA'd (fins no longer attached to main pipe, no leaks though). This means I have decided to get a 280mm AIO now, but my Define 7 Compact case will only fit that AIO on the front.
Current system: 3 x 120mm high flow intake fans on the front, 240mm AIO at the top as exhaust, 120mm exhaust at the rear.
New system: 280mm AIO front intake, 120mm high flow rear exhaust. I'm not sure whether to change the top panel to the closed one, or add a 140mm as exhaust there too. I have a feeling as the case flows ok, I will be alright temperature wise with just the 280mm AIO as intake and 120mm as exhaust.
I think I'll try with the 120mm exhaust on it's own with closed top panel and run a benchmark, and then test with a 140mm in the top and the top open to see if it makes any difference.
AIO I had was the Alphacool Eisbaer 240, new one is the Fractal Celsius 28+ Dynamic (no ARGB headers on my MB, so no point going for fancy lighting).