Depends on the monthly rental. You can usually pay for one yourself with about 1 - 2 yrs rental. But if you rent there is never any finger pointing if there is a problem. You might get a better unit than you provider supplies but if you rent and better equipment becomes available most providers will upgrade for free or for shipping.
Someone mentioned cable sharing. Your provider will most likely install a cable splitter just outside of the box leading into your home. This will insure you get a good strong signal to your modem. If you can insure that you aren't 3/4 splits down from the incoming line you should be fine. I have been sharing my modem with a TV in my office for a couple of years and its has been working fine.