There really isn't a perfect solution to the question, so here are the Pros and Cons of each option:
You are normally provided a new modem.
You can swap your present modem for a new modem, usually without any questions asked.
You don't have to spend $70+ on a new modem if your leased one breaks.
You have to pay a monthly fee.
You are at the mercy of Comcast as to which modem you get.
The ability to choose your own modem.
You don't have to pay the monthly fee.
Retail stores will offer a free or discounted modem with a self install kit and a service subscription.
The modem costs $70+ upfront
If your modem breaks, it's your responsibility to replace it.
Whether you lease or buy, Comcast will provide needed firmware updates as long as the modem is on the approved list: Approved Modem List
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.