Who wins? It's a tough matchup.
Bellatrix was extremely powerful. She could take on Luna, Ginny and Hermione at once and was even able to block on of Dumbledore's spells.
Snape, however, was able to fool Voldemort for YEARS! Making him impossible to read, which was a great advantage in a duel.
Time Period: Second Wizarding War (Battle of Hogwarts)