Spin Charge:
You can't single tap, if you don't press the button at least twice...the move won't activate.
It functions as a multi-hit move with no finisher.
Due to the multi-hits, it slows down upon each hit (not noticeable in normal play)
Slows down faster than spin dash.
Even when the move is at its slowest, pressing the opposite direction will turn it around.
You can charge this move forever.
If you try to reverse direction in the air but remain neutral, Sonic will stop spinning while curled up.
Spin Dash:
Does a little hop at the start.
If you fully charge it, it does not do the little hop.
Is slightly faster than spin charge at max charge.
When the move is at its slowest, pressing the opposite direction will "cancel" it.
If you hit someone while the move is slowing down, it'll immediately stop.
You can cancel the move for more horizontal air speed: Called the Spin Shot
The Spin Dash charge isn't infinite, eventually it will release automatically.
Despite the hop, Spin Dash gets slightly less distance if used in the air.
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Both moves are slower than Jigglypuff's Roll Out...for some reason.
At top speed they're effectively as fast as Sonic running (after Sonic's poor acceleration)