I make more financially with PT spots because I charge more for them but I agree with you that having the same children everyday has huge advantages and there a good reasons for FT having priority over PT.
Less cubbies / space needed to store their stuff
Less parent to interact / communicate with
Consistent group energy
Consistent environment (i.e. I have to change mine every other day as I have one child 2-days a week who puts things in his mouth the rest of the gorup does not and can handle and likes small items like legos, etc.)
I would be honest with your PT family and give them the choice of increasing days. If they cannot / do not want to it gives them plenty of time to make other plans as PT care is not easy to find. The only downside I see to this is they may pull sooner than August if they find something sooner - but you would also be getting 50% tuition for the infant so that may not be a loss financially.
Although I make more financially I feel FT will trump PT going forward in my program as it makes life easier on me. In your case it also is better financially.
Best of luck!!