Having trained in a driviative of Ki no Kenkyukai (Shin Shin toitsu) which is Ki Aikido as I stated above and Aikikai. I'd say the differences are just in the feel. I've sparred some very big strong and light Judoka, I would never really say that Judo is a hard style.
I cant think of any martial system that fits into either bracket. Boxing could be considered hard, but then look at all the evasive foot work, not exactly the mark of a hard stylist.
Then again, you look at those Hsing I guys, that can be pretty hardcore training and they are supposedly soft.
And BJJ, where on earth do they fit. Any 70 + year old man (Helio) that can hold down a 21 year old Yoshinkan 1st Dan (Mits Yamashita) seems very Aikido to me, but then those 2 arts could not be more different.
I think a more usful bracket is, B