The SBG6580 is a 8x4 docsis 3.0 modem. Its going to only have the most base level speeds. Its supposed to be “up to 343mbps” but most isp’s certify it for less than that. Its only a N300 device. But it does have 2.4ghz and 5ghz but its switchable meaning its not dual band at the same time. You have to choose which one you want used.
https://arris.secure.force.com/consumers/ConsumerProductDetail?p=a0ha000000TnGGlAAN&c=SURFboard
The SBG6900 is a 16x4 docis 3.0 modem with “up to 686mbps”. But again, most isp’s aren’t going to certify it for those speeds. But it is dual band concurrently (both work at same time) and is a AC1900 device. (AC wireless versus N with the other).
If it was me, I’d be looking for docsis 3.1 modems. Most ISP’s are requiring docsis 3.1 for lower and lower speed tiers because it takes a load of their network.