First thing to remember is, as someone already mentioned, the whole 16GB won't be available on the phone.
So, first consider software updates and the memory they will need, as they are important. As more features are added to the OS the updates might get bigger. Others have mentioned cloud storage etc, and there are some very good gadgets on the market now for external storage, so photos etc are covered. Next consider how long you will keep the phone for. A year, two years, three years? Phones now to many are a major purchase and many keep the phones as long as they can. If you are a three year investment buyer, will the 16GB last? And possibly lastly, your usage. Now you might be a light user. In a year's time you might be active on here, wanting to try virtually every app you see, you might change jobs and need your phone to run a new business etc, you might join online messaging or gaming groups needing phone use. Life now might not be the same as life in one year, two years, three years.
My rule has alway been, consider what might be and not just what is now. Consider how usage might go up. And even if you don't think you need memory, power, etc now you might in future, so always get the most you can afford.