I can assure that to equality of so large the white shark wins, the white shark is but powerful and fast. One orca solitary is not known to defend of a white shark of great size, the only case in that one orca I kill a white shark was in the Farallon...
Considering the size difference, and the fact that many orca feed primarily on seals and sea lions - some of which are larger than leopard seals, I'd put my money on the orca.
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