Is Late Surrender still an option?
If it's an option, it's best to take insurance (assuming the true count is >=+3), and if you lose the insurance bet, surrender.
However, if they don't allow you to late surrender after the insurance bet is resolved, the math changes. It would be a matter of calculating your +EV on the insurance bet and comparing that to the difference in your EV between hitting and surrendering your hand. I don't know the exact numbers, but my instincts tell me that you'd still want to take insurance. I expect the closest decision would be right at TC=+3 since that is when the EV from taking insurance would be at its lowest.