I had it working well with both.
The only reason I switched from Comcast to AT&T some years back, is because at that time, in South Florida, every time it rained, the cable went down and the internet/VOIPo with it.
Comcast has improved greatly over the last few years and as soon as AT&T starts their price changing again, I will go back to Comcast.

I will assume that your Comcast will offer at least the speed that you now have with AT&T.