Unless you are planning on using Immigration services during your trip, (extension of stay, etc) I personally wouldn't even bother registering at all, not even on the first night.
If you can do it hassle free on your phone, fair enough but if you have to spend half a day at Immigration for such a short trip, it's not worth bothering about.
As your trip is only 15 days long, I presume that you don't plan on a trip to Immigration for their visa services.