I can't give a firm answer, as it will depend on so many different factors. Unless you have MixEmergency's cache set very high (which I don't generally recommend), the 5 second video is probably not all going to fit within it, so you won't really achieve much more efficiency by having it that size. It will also depend on the video codec that is being used.
For the most reliable/smooth playback, assuming that it's compressed using h.264/h.265, and you've made sure that the maximum keyframe distance isn't more than around 20 to 30 frames, then I would probably go for the 5 minute video. It should be less likely to cause resource usage spikes (when a shorter one would when it loops around), and you're less likely to have any stalling on looping.
Small loops are mostly fine, too, though.
