I work for FiOS, but live in a Comcast only area. So disclaimer, personal views.

Most of the close areas of town should be FiOS eligible, unless the building or borough hasn't let them in. Verizon runs a dedicated line all of the way into the residence, where Comcast is just tapping into the main line. Long story short, worst case is Xfinity can slow down when a lot of people in your building/area are all doing stuff online at the same time. Another factor is line condition in your area. If the outside lines are shit, your service can drop. Knock on wood, my comcast has been mostly reliable for 4 years.

The big difference, as stated by annother commenter, is the upload speed. Cable isn't able to give symmetrical speeds like fiber. Thus I am stuck with 1000/40. This will not make a lot of difference for -most- people and they will never notice it, but if you do heavy uploading, such as filehosting, livestreaming, or multiple people VPNing for WfH. you may feel it.