If it was me choosing, I'd be choosing between the c7000 and the c7800. the c7800 has both the wifi benfits (that you won't really see) and the benefit of having docsis 3.1 (which you will see if you have potential for gigabit).
the other option (that I'd recommend) would be buying a seperate modem/router device. When you buy combo devices, the ISP controls the firmware. when you seperate them out, you at least control the firmware on your router. Plus if something goes wrong, you're only out a single device instead of needing to replace the whole combo device. for a little more you could get a cm1000 and an R7800 and have a nice combination.
basically the same modem but the C7000 (600 Mbps + 1300 Mbps) is ac1900 while the C7500 is AC3200 (1000Mbps @2.4GHz with 1024 QAM support + 2200Mbps @5Ghz 11ac). Quite a few devices don't have 1024 QAM support. That's why i said I'd go for either the C7000 or C7800. The c7800 has the upgrade to docsis 3.1 which enables gigabit connection back to your isp.
