Fox will most likely be at least lower Mid Tier, whereas Sonic will only get to higher-Low tier. Fox has everything Sonic needs:
Stronger Attacks
Better Aerials
A Projectile
A Reflector
Projectile necessary?
Not really
yeah it helps but only if the projectile is good and Fox' s projectile is average.
Look at MK.
Better aerials?
No.
The only thing that Fox really has over Sonic is his Dair and Nair.
Sonic's Bair has more priority and much less cooldown time.
Sonic's Fair doesn't stay out as long and can be directly comboed into by several methods.
His Uair breaks as massive amount of high priority moves.
he also has much better aerial speed so he can land his aerials much more easily.
Reflector isn't necessary either since sonic can get by projectiles very easily via spinshot, spincharge, dash shield etc etc.
Its the fact that Fox can kill much more easily and can link into his kills and that his punishment is harsher than Sonic's so a mistake against ox is more likely to cost you a stock than when facing Sonic.
Sonic can approach. Fight aerially. Approach, edge guard, recovery, pressure and chase better than Fox can.
Fox has what matters most in this game, the ability to link into his kills so that he can make those risks and so the payoff is much bigger.
If you look at all the high tier characters (with the exception of Pit) they have various methods of linking into a kill move.
Sonic only has one.
spindash to bair. And that can be avoided too with good DI.
Stat wise Sonic beats out Fox, he beats out a good number of characters, but that issue with killing means that he just can't get the job done quickly enough to make all his hard work matter.
If i use Fox, I can Ko the opponent at around 110 %. Yeah if I screw up my approach I am definitely going to get punished. But hey does it matter? If I am successful I can KO my opponent early enough so that it equalizes.
With Sonic yeah I can avoid harsh punishment but even if my opponent is at 150% it means little if I have issues finishing the job.
****IT ROY BEAT ME THIS TIME! T_T