Your cable system, like over the air TV, probably only supports 720p and 1080i. You can check the set up menu of the xfinity box to see what options it gives you. In general if you have to choose between 720p and 1080i the progressive 720 will be better on most things but it does require more bandwidth that 1080 interlace so most channels will be 1080i.
On a UHD TV the TV will have to perform additional processing to upscale to 4k. This will affect the quality of all non 4k programming so if you see artifacts you may be able to turn some of the processing off but they may not be avoidable.