What is the ideal brake pad material for a MTB used in both downhill and cross-country/Single-track settings?
Metallic would be better as you are suffering brake fade. Advantages any other type are far outweighed by avoiding brake fade. 

If you have the option when you replace the pads, look for ones with a heat sink if they are available for your calipers. Suggest you should look at your riding style, and especially see if you can pump the brakes rather than drag them. If needed, a larger rotor will provide better cooling for a relatively small cost (rotor and adapter)

There are two causes of brake fade - one is pads/rotor overheating and loosing effectiveness, the other if boiling brake fluid. It may be worth bleeding you brakes, making sure you use a high quality fluid - if DOT, make sure you use a DOT5.1 which has a higher boiling point than DOT3 or DOT4. If mineral, there are no standards as with DOT - I would use the fluid recommended by the brake manufacture.