15$ a month is 360 over 2 years.
it's 720 dollars over 4 years.
yet still only 15$ a month ...
i wouldn't want to have to limit myself in any way on the device i use the most throughout my everyday life because of a meager 15$ a month.
save those 15$ on something else, not on your smartphone.
generally, i'm not a big fan of these kinds of calculations.
if you buy a can of soda for 50 cents each day, that's 365 dollars over 2 years ... it's still just a can of soda every day.
but then again, i don't know about your financial situation.
but then again, if +/- 15 dollars each month would be a significant change in your disposable monthly budget, i'm not sure an iPhone is the best device for you to start with
that + what others said about having unlimited but not needing it being better than not having unlimited and needing it some day.