Thermal PCB design is always an interesting problem. These days there are many solutions dependant on the circuit complexity and how many layers required to get the routes in. I recently did a 12 layer (all 2 ounce) flexi-rigid layout with numerous D pack and similar devices, lots of copper pours and thermal vias. The best use for metal clad is simple circuits preferably single layer routing where you have the lowest possible thermal resistance down to the metal. Generally for minimum cost simple LED boards on single sided layout offer the cheapest option, then you can get as complex as you like, but this does come at a cost, maybe thermal vias with an external heatsink attached would be a less costly option, lots of methods out there.
How complex is the circuit, what is the PCB size etc. all these factors can have an influence on your design choices...