For NASA, the spaceplane never goes orbital and gives you a jumpup on Mercury, if I understand it correctly. But the PKA can go orbital, so it can give you the first man in space, etc - but is useless after that. The PKA, unlike Gemini, Apollo, Voshkod and Soyuz can't be used as part of a lunar mission. So you either do the PKA instead of Vostok or not at all.