To publish or not to publish a paper in an ERA "type C" conference?
You should have asked yourself this question before you submitted to the conference. Withdrawing your paper now, after it's been accepted, means you wasted a bunch of people's time (editor, reviewers, ...). Do unto others ...

Additionally, given that it was accepted as a borderline paper and was rejected at two previous conferences may be a hint that your work is not perceived as top of the line by external referees. I doubt you can shoot much higher than what you have now.