You're right, it's probably better to use one IS31FL3733 than two IS31FL3732 or IS31FL3731 at the bottom of the PCB... it's optimized for RGB unlike the IS31FL3731/IS31FL3732.
I'm using IS31FL3731 in my keyboard PCB, same matrix size as the IS31FL3732, but 4x4mm instead of 5x5mm.
There was a definite advantage of using two IS31FL3731 in the middle of the PCB and splitting up the RGB LEDs into four separate matrices. I was able to route this with only two layers and 10mil tracks, and also squeeze an extra four RGB LEDs per driver, bringing the total to 72 RGB LEDs... good for a 65% PCB as well. But the advantage only comes from placing them in the middle of the PCB. If you were forced to place the LED drivers on the bottom, then the IS31FL3733 is better than two IS31FL3731/IS31FL3732, less tracks, maybe even possible with two layers.