Arsenal 5.45 vs 5.56

Gunnit:

I am about to invest in a SHTF firearm. I am going with an Arsenal SLR or SGL. I have an Armalite, but i leave that at home (i am at college) for various reasons, so i wanted something i could keep at school.

However i can't decide what caliber. The 5.56 would be easier to find, however i can buy 5.45 in bulk a little bit cheaper.

Opinions?

EDIT: i live off campus and this probably would rarely leave my trunk except for the occasional shooting range trip.