Just for the 300mbps service. The other modem will get you close to that, but the 6183 will consistently perform.
They only "sounded" knowledgeable if they recommended you those products. The modem is probably okay, but that Tenda is not going to reach its break even point...
For the same price, I'd really recommend this bundle http://www.amazon.com/SURFboard-SB6183-DOCSIS-NETGEAR-Gigabit/dp/B016P8RHHO