Bottom line D.J.: If you are using internet & voice, there is little advantage to buying your own eMTA modem...too many restrictions from Comcast, and they are not all that cheap, but if you are getting internet only, buying your own modem makes a great deal of sense.
A very good modem such as the Zoom 5341J can be had at retail for $79 - $89 which will quickly pay for itself.
The only authorized self-owned eMTA, the Arris TM722G, while being a good enough unit, is already a couple of model years old, and at $149 & installation fee, is a less compelling self-own solution. By the time it makes the "paid for itself" category, it will be a 5 year old model...