The price of modems have come down in price over the past year or so. On one level, it makes sense to purchase the modem as you'll recoup the cost in just over a year. However, if the modem goes bad, you're responsible for it.
Renting has it's advantages as well. If something goes wrong with the modem, all you have to do is get the provider to change it. Even if the modem gets fried by a surge in your home, you'll find that you can change it out very easily.
It comes down to personal preference. Do what you feel is right for you and your situation.