In a SHTF world, if there was a Mini-14 and a AR15 sitting side by side - and I could take either - the AR is in my hand every single time. The reasons why are somewhat technical, while another part of that statement is just my preference. Before I would hazard an answer to "would it make sense to swap," I would have to ask a bunch of questions about your specific situation.
As for the Sig M400, that is a good price as far as I can tell. I saw two sellers on the web at about the same, but after shipping costs, the Walmart deal looks solid.
Let us know if you buy a new blaster. Always interesting in hearing how things work out.