You need to make sure your modem supports 3.1 too, don't forget that.
Cox has just recently finished upgrading to all the new 3.1 hardware here, and phoenix tends to be their technology leader market due to being their biggest, so I'd be surprised if comcast has done more rural areas. They always seem to be the first to fight any sort of rural network legislation as they hate wasting their capital on non-rich areas, so make sure they can even support the 8-24 channels down needed for those speeds. You'd be amazed how screwed up cable plants can be in rural/old areas that in some cases, they just cannot support the rates. Tempe was like this for a long while here due to the original podunk cable co that built it.
Cox is actually one of the better cable MSO's out there sadly. My first job in tech in '99 was @home networks that pioneered cable modem tech, and taught the cable behemoths about that little internet thing, and dealing with them all from comcast, cox, intermedia, att, and others, cox was always one of the least crappy of them. So much I even worked for them after for a bit (more crappy to work for imho). Not perfect, but definitely better. If you like to pirate media, oddly they were one of the strongest to reject lawsuits, abuse subpoenas, and other media cartel incursions on human rights, where comcast (being the ultimate media whore thanks to corrupt/owned fcc) is the opposite...
That said, I've been having more frequent outages (including 2 yesterday) with Cox, and they're steadily warning and charging me for bandwidth now, so I'm having Centurylink installed to check out that is almost double the speed and $25 less than my cox bill now. As much as I hate Centurylink, they don't have the bandwidth caps, that started with Comcast coincidentally, and Cox can stick it now (pun intended) that they impose them too if I'm just going to go over monthly. Joy of having both services at least temporarily is I can steer traffic out either/both with my Fortigate firewall with sd-wan features, so going to play a month or two before I can one or the other to see...
-mb