The Lapras for sure. The 150 or so extra stamina it has over the Cloyster more than makes up for the ~130 defence and the extra IVs that your Cloyster has over the Lapras in survivability, because IVs make the least difference in combat compared to Pokemon species and moveset.
Cloyster is more potion efficient but that's about all it has over a Lapras. Potions shouldn't really be an issue anyway.