I have both cases, the Porter case seems to be more roomy and is handier when it comes to airport travel (as a carry on), as you can used as a dolly, the footprint is also smaller. The 1550 pelican is steardier, specially if you get the one with the foam padding, but is smaller.
Normally I prefer the pelicans for travelling with delicate gear but I don't know if you can accomodate a 4x5 and 5x7 on a 1550, maybe on the next size up, of course at that point is not carry on luggage anymore.
Hugh