Academic year salary is generally the hardest thing to negotiate, so do look to other factors as well.
What is the length of these two postdocs? With a two-year postdoc you need to start actively applying for your next job about 14 months from the start. On a three year post-doc you can apply for a few great jobs at this time and then apply more widely during the final year. It might be possible to get a two-year offer extended to three.
Summer salary can sometime come from a different source. If the salary is an academic year salary, you can ask for some summer salary. It is possible this can be found from another source than the grant funding the postdoc and is a way around the limits on the grant.
Moving costs. For a short-term position, this can be important. If you move as a postdoc every two years the moving costs can be brutal. If they offer no moving costs, ask for a few thousand dollars, perhaps.
As other's mentioned, do these require you to teach? It is possible the grant funds are supplemented with teaching funds, and these may come from outside the grant. They could be more or less flexible.
Travel funds? If you want a permanent job, you really will want to go to conferences. Look into other research expenses. Do you write papers with someone else? Can there be funds for them to visit?
As to the cost of living, there are online resources to sort out costs of rent and food. As to health insurance, ask the two schools what portion you pay. University jobs can have surprisingly large deductions out of the paycheck and these are not uniform across schools.
Grants in the US are not like bank accounts. Moving funds between categories is subject to opaque rules. You want to prompt the PI on the grant to think creatively of all possible ways to get you money. I suggest you signal to the PI that you know rules on funding are tricky, and suggest you want to work with them to see if they can hire you as a postdoc without your going too broke too fast.