The first time I fought a sonic, I was pulling out my hair. I was indeed too slow. Since then they've been, on average, even fights. Sonic is now one of the fights I look forward to the most because of the challenge of avoiding being hit and landing hits while having lots of time to adjust to each other. Very fun fights.
I recommend a lot of stomping until sonic gets into the air. Then mix it up with aerials. This is a good fight to learn to not leave openings because you'll get punished so fast it will make your head spin. Choking is great if you can land it, but once again, be careful about leaving yourself open. I think Sonic can cancel their spin dash into a grab if they jump into you and you're shielding, so make sure to drop the shield if they're blooping in from above while in spin dash mode and punish with an aerial, preferably with u-air. If you do get grabbed, make sure to tech the down throw or you'll get *****. That isn't to say they won't use other throws, they're all pretty situational, but just make a special note of teching down throw to give you enough reaction time to take on whatever he does next. If you get knocked off the edge, be ready to u-air Sonic if he comes out to get you. Otherwise you'll get gimped.
Oh yeah, his up-b doesn't sweet-spot. Don't go for the gimp, go for the kill. His up-b allows him to cover too much distance for gimps to be applicable. Stomp his *** or, my favorite for his recovery, u-tilt.
Sonic's f-smash has deceptive power and range. His u-smash is negligible and the d-smash is stomp fodder. Tilts are very quick and have good range, but your d-tilt is better. Also, be sure to d-tilt your chokes for more chokes, d-tilts or u-airs.
This probably goes without saying but this match-up goes to whoever punishes the other guys mistakes better. Sonic is hard to punish but Ganon punishes hard given the chance. Ganon is easy to punish but Sonic doesn't punish very hard. I'm almost inclined to say this match-up is 50-50. Certainly not an advantage for Ganon, but once you learn the fight, I don't see much of an advantage for Sonic either.
50:50.