The following first website shows how to buy an iPhone 6s or 6s, plus four other websites which compare carrier plans to purchase one immediately or over time, or lease one:
Ultimately, the financial impact on your budget is largely the same, it just depends on whether you want to spend the money now or stretch it out over a long period of time (and whether you want to keep the phone to give to others, or trade it in annually for a new one). The choice is still YOURS . . . !!!