Uh, take what I say with a grain of salt since I'm no vbrawl Sonic player,, but..
Sonic feels AMAZING in projectM. And I really like how the vB and >B work. In vbrawl, I didn't even know there was a difference between the two moves.. lol But now, I just find it so much better now that I distinctly know that vB is for spinshotting and offensive use while >B is for recovery/ midair mobility. IDK, again, I'm not a vbrawl sonic player, but playing vbrawl sonic I had no idea there was even a difference between the two, and always questioned why they put the same move on two imputs haha
supahsanikracinn
Well, here is somewhat detailed post on the two:
Once side-b and down-b hits the ground, they are basically the same move. The only differences is how it starts/charges, how much damage it does and how they act in the air at the start.
With down-b you can charge it for more damage and speed by tapping b as you charge it and it can cause more damage in the air descending on someone if you hold forward which can also be canceled into shield descending on the ground . It can also be canceled into shield from charging it on the ground.
Side b can no longer be charged but if you press b quickly enough like a hot potato, instead of hopping it does an instant spin dash which is faster then downb but wont do as much damage. The hops height depends on how long you press b when you first do it on the ground. In the air, the hop has the same height all the time. With the starting hopping part, it has little knock back that can lead to a back air or footstool or something like that depending on how much % opponent has and your position. In brawl, the start of the hop had some invincibility frames to break through some attacks. Im not sure if this is still there.
There is a third thing called spin dash jump, which is done by jumping from the ground part after you release any of the 2 spins which can do more damage if you can land the ground hit then the jump hit. This works better on larger targets.
With melee airdodge, you can do a wavedash or airdodge into ground from the sideb hop or from spindash jump. This can have you stop the spin very fast or if you hit with the ground move then wavedash into a smash or grab or something else.
And with recovery, the sideb can help with distance because of the hop but there were times when it when down at an angle at a fast speed so its risky. Of course you can also use downb and with saving your double jump, you can spin to get horizontal distance after knockback wears off then double jump, then spring or homing or airdodge.
I left out Spinshot but Im no good at doing it. I might have forgotten something but thats all I know.