One lives, the other doesn't. Whitecheek Tangs have a pretty bad survivability track record. Most tend to waste away in aquariums. Rejecting food, stress from too small aquariums, etc. Matches it very close cousin, the Achilles Tang.
Powder Browns are also commonly available smaller, which is preferred. A PBT should definitely be one of your last additions. They are pretty aggressive once established, even by most tang standards. Usually claiming the whole tank as theirs.