Right way of wiring discrete LEDs to make a high power LED panel
There is a resistive component to the LED I-V curve, which is especially significant at higher currents. This helps in making the current distribution more even.
The Vf of the LEDs will vary with temperature so you might want to connect them in a way that the inevitable higher spot temperature in the center of the plate does not unduly influence any one string. This may not be so easy (or even possible) on a single-layer aluminum core PCB. Without an aluminum core PCB and ~75W of dissipation (the remainder leaves as visible light), your PCB may tend to self-immolate.
Chances are your LEDs will mostly be very close to each other in Vf at a given current because the dice are taken from a wafer in a very organized fashion in most cases.
A constant current to the series-parallel array makes the most sense to me.
You could also consider adding to the series resistance of the strings by inserting 25 physical resistors, one in each string (still driven with a constant-current source), but that would add to the power dissipation and you might want to figure out what the intrinsic resistive component is to estimate the improvement.