Voluntary sector may not be fully state funded. Rates of pay on a similar scale may but be as high as the highest point in the salary might not be possible with an equivalent position. It might also be a springboard to getting another public sector job or voluntary job at the same or higher level with new skills. If it’s your ideal job and you are not tied to the public sector then it maybe more satisfying for you. Just research the organisation, its funding, your boss.