I have wondered a lot of the nether on the latest beta and I have seen that the most common biome is the nether waste with a huge diference than the other biomes (Its rare NOT to find the wastes in a nether) and soulsand valleys being very rare compared to all the other biomes. In fact i could say they are so rare that after traveling on the nether of a world for 1000 in any direction i only found 2 areas of soulsand valley and a big area of nether wastes. I dont know if thats intendet to be this way with the rarity of the biomes, but i think its not.