Does Comcast provide a free (or less than $5/month) cablemodem? If so, that's the best one. Some package deals will include cable modem rental. Mine was $1/month for about a year and a half.
There is a benefit to using the cable company's equipment when it's reasonably priced. It puts their responsibility one step closer to your computer. If you use your own modem, their responsibility ends at the end of the cable. If you use their modem, their responsibility extends to the cable modem as well. Seems like a minor thing but it can make a big difference in the event that you have trouble with your connection.
If they charge an arm and a leg for the cablemodem rental, look around at the local electronics and office supply stores. Someone will almost certainly have a cablemodem on sale for half the normal price. Office Max has a Best Data modem for $50 after an instant (not mail-in) rebate. I saw another brand on sale for $40 at Best Buy not too long ago. Don't worry about the brand too much. As long as it's DOCSIS compliant, your cable company should have no problem with it. Doesn't hurt to call them first and ask, tho.