You can apply for FIS while on Maternity Benefit, as long as you are not a qualified adult on your husband's claim.
If your husband moves to JA, then he will only receive the difference between the max rate for your family and what you already receive in Maternity Benefit, and then will have money assessed off him as well, so you would not be better off. You would also have difficulty getting him back on to JB in the future.
FIS is probably the better way to go. Usually FIS lasts for a year, but if you get it while on Maternity Benefit, it only lasts as long as your Maternity Benefit.