Every time You get blow to metal its or broking or deformated. In that way wood usualy start to broke. Thus metal brings all power of blow straight to shield. Leather if its thick and moderately soft will make blows softer. So its better. I recognized it in practic. Im from NorthEastern Europe and think You know that here we fight much more hard. Especialy if in festiwal Coming some Polish, Russian or Ukrainian. Leather shields prowen their riability - metal borders are broken easily.
I have made already 7 shield from planks, not plywood, for myself and for sale. They all are covered with leather 3 -5mm thick. The oldest have 3 years and its still usable (I was finghting against halberds with him too. In one year I have usualy 10-13 festivals) . Maybe he is not so pretty as in begining, but its still very reliable.
Any way - You can try to use metal. But You will change mind if You ever try good leather
also, how I envy your festivals there, in quality and amount... here we luckily get one or two in a year, and not even close to the ones there (I hope that with time and hard work we could change that)