If rotors have no marking on them then you can presume they are safe for any pad material, then you decide the material for your riding use.
Pro's & Con's of material types and use are all over the web so a simple search will help you with your criteria.
I tend to buy pads (pairs) cheaply (< £1.50) by the half dozen or in ten's and stick to resin/organic predominantly. I have tried metal/sintered in the rear brake though found them ineffective compared to resin.
The only issue with resin is they wear badly with offroad wet condition's esp chalk/clay mix, I did a pair in under 30 miles of use on the South Downs.