Not a Number's advice/links are great; use them. Also keep in mind that if you buy your own you are responsible for any technical problems you have with it. Yes, it's easy enough to get a new one if it dies, but do you have the know-how to diagnose it and know if it’s dead? What if the settings need changed or get set incorrectly; do you know how to check and fix them? Leasing a modem from them should get you support with it as well.
Also consider the costs. If you only plan to be in the area, or stay with Comcast, for 12 months, then it only cost you $36 for the modem. If you decide after two months to try a different provider or technology, it only cost you $6. That also saves you the time spent specing out good ones, time spent trying to find and buy one, money spent on shipping, risks with buying a used product, etc.
I would also advise you check into speeds with local users as well. Cable modem speeds are inherently volatile. If a lot of people in your neighborhood are online, for instance, you may notice a serious drop in bandwidth. Also, while they usually offer speeds of 8 Mbit down, they will usually restrict you to 128 kbit up...that's extremely slow. Most people use down way more than up, but if you upload pictures for printing, play games, etc, the up speed is going to be important and 128 is a snail's pace.
SDSL will get you anywhere from 256 kbit both ways to 1.5 kbit both ways. The former is probably what Adnedarn had experience with. The latter will probably feel as fast as a cable modem coming down, and it will be much faster up, more reliable, and more secure to boot. Saying DSL is fast or slow is kind of like saying a car is fast or slow. A 'car' could be a 80 horse power Prius or a 600 horse power Ferrari. The same goes with DSL; make sure you know what speed you're getting before you buy (good advice for any kind of internet connection)
Verizon is also pushing their FIOS service around here, which offers either 2 Mbit both ways or 5 Mbit both ways. It's a ton faster than cable or DSL and cheaper in our area. I would personally look into this if you're going to spend that much (right now I just pay $19 for the super basic 384kbit ADSL).
The dslreports site that NaN linked to has speed tests; use them to test a neighbor's cable during peak hours if you can. Use that site to also learn about internet connections so you can make an informed choice.
Good luck.