I work for Comcast and take part in pushing these new wireless gateways on our poor customers who have perfectly good, stable modems in place. My advice is to steer clear. If everything is working fine then why mess with it? Comcast has us pushing them so hard because they want to build their Xfinity Wifi public hotspot network. Not really that big of a deal considering that the bandwidth for strangers connecting to the hotspot is so limited that it has negligible performance impact to your actual in-home network and there really isn't any additional security risk at all. The problem is that our wireless gateways are built cheap and have a much higher failure rate than any modem we've leased in the past. There was even an alert sent out to employees a couple weeks back about a design flaw where the coax cable can become loose from the back of the modem and cause an ingress signal leakage which severely harms both upstream and downstream bandwidth and the solution offered by Comcast is to advise that "tightening the coax connection a quarter turn should resolve the issue."

As far as purchasing your own modem, I say go for it. At $10/month for your Comcast rental, it'll pay for itself in time. The SURFboard SB6141 you posted is a fine choice, though I personally prefer the Arris CM820 or if you have phone service with Comcast then the Arris TG862G (you'll notice the modems with phone ports are quite a bit more expensive but what can you do?) The only reason I prefer these over the SURFboard models is that they're built like fucking tanks. I guarantee either of those two would keep working until they've become obsolete and then even some time after that. I picked up the Arris TM722 new off Amazon more than 5 years ago and haven't had an issue with it yet. One other thing to note is that if you do pick up a modem that has built in wifi, expect to disable the wifi on it because I can almost guarantee that your dedicated wireless router will have a stronger signal strength anyway.

Lastly, this may not happen to you, but when I made the switch to my own, purchased modem and called in to Comcast customer support to have it added on and activated on my account, it took four or five calls before getting a rep that was competent enough to know how to even enter the serial number onto my account. More than one agent told me after trying for some time that they don't support that model, but it's the same fucking model they were actively leasing to their customers at the time and is even on the list of Comcast supported internet modems so don't get discouraged if you hit some static while going through that process. You will eventually find an agent that knows what they're doing. Anyways, best of luck with your ongoing relationship with Comcast :)