I think the answer you're looking for should be determined by things other than head pressure and lift, since for all practical considerations they are virtually no differences between top of bottom fill.

Top fill will allow you to pump and run better. You fill until the tank overflows, kill the pump, pull the suction and run, no vavles to have to close (and w/o valves no maint of them either).

Bottom fill will allow you to back flood the pump to help the prime, but you'll need to remeber to shut the fill valve once done, otherwise you may end up getting to the scene and find the tank half empty.

Other consideration, an outside 4" fill pipe (to the top) is more likely to freeze on a long haul, so that may argue for an internal pipe where the entire tank would have to freeze first.

I imagine you'd want a quick drain system to dry out the pump in the dead of the winter.

On the other hand, an internal pipe (bottom or top fill) will be subject to corrosion where it penetrates the tank (assuming steel not alum or poly) and will likely be the first place that rots through.

One thing for sure, 4" pipe with an external fill for filling from an engine will fill faster than 2 1/2 or 3".

SO question, how do you keep your trucks from freezing while they are parked in your FD's igloo?