Hello Anon I'm a mere learner but I'd say either is OK. OED suggests "which of us" is commoner than "who of us", and indeed if you google, you will see the ratio of "which of us" users to "who of us" users is 20 : 1 in UK sites. But in America, the ratio is only 3 : 1 to 4 : 1.
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AnonymousWhich of these is correct? Important: I need a grammatical rule.
"Who of us would not be afraid?"
"Which of us would not be afraid?"
(my boss misuses this all the time in his writing, and I would like to have a rule to give him for the correct usage.)