It will depend on your rate of PPP.

I am on PPS and it is better for me to stay on it, rather than switching to AUSTUDY purely based on the amount.
I *do* use JET and it is definitely worth it, particularly if you have more than one child.

My advice would be to stay on PPP and access the JET if you need it.
Are you sure you cannot access JET on AUSTUDY? That seems odd to me.

Briggsy's Girl should be able to clarify that for you.