Sonic in my area is basically using AT&T line, so no experience with Sonic fiber optic.
It's always a switching game for me. These companies rather get new customer than keep existing customers.
I have had good experience with Comcast. I guess I never had issues and had to contact Comcast customer service. One reason might be that I use my own cable modem. Most recently, my intro offer was $30 for 75mbps, but good for 1 year only, and that ended just 3 months ago. After intro is over, Comcast offered some discount, but not even close to intro offer. So time to switch.
Then with AT&T U-verse, I have more issues with their hardware. I had to replace the gateway multiple times over the years. As for cost, AT&T is pretty good with offering discount to existing customers after the intro offer is done. So I tend to stay with AT&T for 2-3 years. Once AT&T pricing is not as competitive, then back to Comcast.