I have BB7's. Which pads are more durable: organic, semi-metallic, or metallic. Which is best for touring? Does it even matter?

metallic = most durable
does it matter?
depends on how much weight you are slowing down
the rotor size
and if you carry an extra set...

personally...
I use a 203 rotor up front, metallic pads, and carry 2 sets of pads.

my personal thought is that a bigger rotor gives more leverage
therefor
less force is needed to be applied to the rotor at the pad to provide "x" amount of braking
therefor
reducing the amount of wear on a pad
biggest rotor and metallic pads = most longevity

by comparison, I've used a 160mm front rotor, with organic pads
and yes... i wear those out much faster.

my experience is...
i don't own a car
the bikes i ride are
a custom Hunter 29er with (f) 203 (r) 160, metallic pads
Surly Pugsley (f) 203 (r) 185, metallic pads
Surly Big Dummy (f) 203 (r) 185, discobrake Ceramic 3xLife pads https://www.discobrakes.com/?s=0&t=0&c=2&p=105&

all with Avid mechs.

between my bikes, I wear out at least 3 pairs of brake pads every year. thats at least $120/yr

braking performance in dry conditions, i.e. mtb, organic pads are "nicer"... they feel softer, and have better performance, but wear out faster...

personally, I like metallic pads.

EBC Gold pads are good too.
the Hunter and Pug, both have Avid Metallic pads, and i like them a lot.

As Asana can testify with his touring experience with his Surly Big Dummy, disk brakes have proven robust and reliable on journeys far from home. There is no reason they can't be used on standard touring bikes.

i loved my avid bb7's on my recent tour of Yunnan Province China..... LONG grades there. Unpredictable road conditions - animals, potholes, abrupt short sections of dirt road where landslides were so common that repaving made no sense, folks herding pigs-goats-yaks-geese, piles of building material, grain threshing activity, rice drying, giant piles of manure for the crops, parked farm vehicles - all right in the roadway. those brakes got a workout. used the organic pads, liked them