Asajj Ventress was a duellist who could handle some of the best ever duellists in combat, Bastila's only claim to fame in her abilities was Battle Meditation, she has never shown any immense skill in Saber combat or Force prowess, otherwise it would not just have been Revan's one-man party throughout KotOR.
Given the dubiously canon nature of many of Ventress's 'OMG' moments, there's no doubt in this match-up. Bastilla Shan may have had a primary function leaning toward directing large groups with her battle meditation but her role in the events following Revan's adventures shows that she is vastly more experienced in Force lore and lightsaber combat,
The Clone Wars cartoon may paint Asaji Ventress in a favorable light but her skills are rudimentary at best and she has no discipline or deep Force connection.
Ventress was a well-honed Assassin, Bastila was a one-power wonder.