COB LED array v high-power LEDs

High-power LEDs (single led usually powered at from 1 to 3 Watts)
Advantages
Creates a small source for the beam, ideal for creating a focussed optical beam using a lens

Flexibility of choices of beam-angles depending on luminaire design

Ability to spread the LEDs widely across a cooling metal circuit-board or heatsink for good heat-reduction to give the LEDs longer life.

Disadvantages
To make more light you either need to drive each LED with more power (which is less energy-efficient, eg a 3-Watt LED often gives less than double the light of the same type of LED driven at only 1Watt)  ...or you can add several more LEDs into the light (which costs more and consumes space)
COB array (Many tiny LEDs built into a single unit)
Advantages
Each tiny LED needs less power as there are many, all creating light and so each LED works very effiiciently
COB LEDs are mounted directly to the printed circuit board and if this has good heat-transfer properties the combination can be good for cooling the LEDs.

Disadvantages
As the light comes from a larger area (sometimes more than 4 sq cm) it's difficult to create a sharp, focussed beam
Heat needs to be dissipated from one small 'footprint'. Heat eventually wears-out LEDs so advanced cooling materials/methods are needed if the array's power is over 60 Watts
Beware, some companies who assemble LED hig-bay industrial lights power the LED arrays higher than the manufacturer of the LEDs recommends. This is only bright as a short-term solution as it will wear out the LEDs sooner.
www.energyatwork.co.uk