Ventress would win. As much as I dislike her as a character, she would win. She has dueled many members of the Jedi Council, including Luminara Unduli and Plo Koon. While she didn't necessarily defeat either of these Jedi in battle, she survived and that says something about her.
Plo Koon is a master of the Djem So form and a very powerful Force User. Ventress broke Koon's right arm by bringing an avalanche down on him. Plo Koon would catch up to her and defeat her with his left arm.
Unduli was a master of Soresu (not to the extent of Obi-Wan) and was able to hold her own against Ventress while her vision was impaired.
As powerful as Bastila is, she doesn't have the skills or power to hold her own against Ventress, who has killed many powerful Jedi.
most of the time with ventress the reason she survived so long was because she was a coward so saying her surviving fights cant really be used
but i do think she is a better duellist but bastila is extremely skilled in the force ( i think the youngest jedi to of mastered battle meditation (something was not seen in any of the films.))