Comcast is probably 250 Mbps download and something like 10-20 Mbps upload. Upload is important for things like backing up photos to Google Photos or iCloud etc., and for generally reliable internet access.
Sonic Fiber would be 1,000 download and 1,000 upload. To be honest, most people won't notice a difference for downloads unless you have a bunch of people using the internet at the same time, and WiFi is frequently the limiting piece past 300+ Mbps. Sonic has many other benefits (net neutrality, good customer support, lower ping latency)