I think the reason one would choose a laptop over a smartphone is three fold.
1: No comparative limit on Overall capacity: Show me a smartphone with a 1tb storage capacity. Granted you might not NEED it, but there is the logic of having no upward celing on such capcity.
2: Compatibility. Be it from the prospect of having a USB host hub built in to charge dozens of smaller devices ......
(provided you have something such as a portable inverter to keep IT powered )
..... to the prospect of interfacing with other digital files out in the wastelan... Err uhm In an emergency situation. Its logical to have a more compatible operating system that allows you more freedom than something like Debian, IoS, Android Solaris, Symbian, WinCE would be able to provide. Plus there would be file type compatibilities to consider. A smart phone has a degree of logic to it yes, but smart phones also have massive limitations in this area.
3: Performance. Smart phones do OK for themselves. However there is advanced software but their software capacity can be limited by hardware limitations. Consider the lauded Iphone is essentially equivalent at best a 600mhz -1ghz processor, 512 ram, about 32 of graphic video ram. Essentially... its a computer... From the turn of the century. So obviously, unless the software is highly optimized, and extremely specific for the operating system, any software is going to have a rough time, and you can essentially forget about multitasking with it.
Now again.. i get those things MIGHT not be needed, but, then again, dependent on what sort of survival based software you need to run, it might well be VERY relevant.
Anyway... Honestly I still think the most important part would be a groundable Faraday case to protect it from what essentially every other case cant. OH! and a 360 controller just in case you get bored.