buying a modem

For 300mbps internet you won't need DOCSIS 3.1 for now you can buy an Arris SB6183 for cheap, its a tried and true modem and a best seller. You can also make the jump to a 32x8 channel modem for DOCSIS 3.0 but there is the Puma 6 Chipset flaws that affect those.

I also recommend shopping for Netgear modems as well. The CM600 is a good modem as well.

The modems I have listed are all more capable of your speed and gives you more rooms for upgrades in the future.

You could also buy DOCSIS 3.1 modems and future proof your home network. But its a roll of the dice when you buy one. Your area might not ever get gigabit so it would never take advantage of the features.

After all the point of owning your modem is to prevent rental fees, and pay yourself back for the purchase of the modem by saving rental fees.

.@kunyach 

 

FWIW, the Comcast supplied device that you mentioned is a combination modem / router gateway device. The devices that others have mentioned are straight cable modems with which you'll need to purchase and use a separate router for your home network / WiFi. Be advised that there are also combo gateway devices available at retail.