What is the ideal brake pad material for a MTB used in both downhill and cross-country/Single-track settings? all! 

I and my wife both spend a lot of time doing both downhill and single-track MTB in New England. We both are still on our stock organic pads, and are looking to replace our pads soon-ish. I, especially, suck hardcore and tend to spend a bit of time on my brakes going down the 5 mile long green downhills. I notice significant fade by the end of the run, and the pads are quite worn down after only a few trips. The wife is looking for more aggressive pads, mostly.

I have reviewed these related articles on SE: 1 2

It would appear that for a single use bike, the answers are more or less self evident - sintered for downhill, semi-metallics for single-track.

However, which pad is best for a mixed use bike? Is it better to go for the more rotor-chewing sintered pads, or the more balanced semi-metallics? I am leaning towards the more aggressive pads - rotors and pads are cheap, but injuries aren't. I'd rather cause excessive wear on ST versus having poor performance on downhill, where I have a knack for getting injured. (Some day my elbow will be enough scar tissue that eating it doesn't result in any new lost skin /s)
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.