Its backwards compatible but if it gets the message from the CMTS (First hop between your modem and ISP) it will still channel bond, and if the signal for those channel are junk, then it will just get stuck in a loop. So yea, unless your ISP offeres DOCSIS 3 modems, don't go DOCSIS 3 yet. Not to say you should buy the modem from them, but if they sell them, then they have to support them, which means they have to monitor market wide conditions on the node that could hinder them.
With that said, when you do go DOCSIS 3, 6120 is the way to go, hands down. Case cooling is better then Cisco, PSU is better, Chipset, firmware, and so on.