I'd only ever use a mifi device if I'm always carrying a bag, or with a lady with a handbag to put it in, if not I'd just buy a sim card at the airport. They arent expensive.

Another important thing to consider is to switch off data roaming on your iphone, so that you dont accidentally run up a large tab if you wander out of range of your mifi device.

I also wouldnt worry for the slightest second about my iphone being stolen in Japan, as for losing it I dont see why that should suddenly become an issue because you're in Japan?