The best way to get your own modem that Comcast can't charge you for is to buy a RETAIL box Motorola SB6141 and then get a separate router.
A retail modem will have a white case, and will have a serial number that will verify as a retail purchase unit, not one sold/provided through a carrier. If they try to charge you rent on that, it's easy to fight. Tell them to verify the serial number, they will quickly find it's not one of theirs. If you take the white case modem into one of their stores, they will quickly see it's not one of theirs. I have used one on both Comcast and COX and neither tried to charge me.
If you buy one in a BROWN box with a black case, it's serial number will be in the ranges sold to carriers (like Comcast). They can claim it's theirs, but you can show them the purchase receipt. I had this experience with Comcast once.
The SB6141 bonds 8 chans down and 4 up, and is good for up to 343 Mbps down.
http://www.amazon.com/ARRIS-Motorola-SurfBoard-SB6141-DOCSIS/dp/B00AJHDZSIThere is a new SB6183 that bonds 16 down and 4 up which is good for up to 686 Mbps down. If you don't want to buy another for a LONG time, that's the one to get.
http://www.amazon.com/ARRIS-Motorola-SurfBoard-SB6183-DOCSIS/dp/B00MA5U1FW/ref=pd_sim_147_5?ie=UTF8&refRID=0SX5VWXE7YDXTA0589ME