If you're planning for this to be a SHTF gun in addition to a plinker, I would go with the 223. In a SHTF situation, the odds of finding components for 300 Whisper will be less likely than being able to find .223 ammo. Not to mention the extra time and effort involved reloading which could be put to better use in an already stressful situation. You can't always count on having access to your stash of ammo/components.

If you are just looking for a fun gun, the 300 would be awesome in an AR. You could look at getting an upper in both rounds.