I think he is left-handed,and I have some proofs: 1)He wears many bracalet-like items(like the crystal ring from SADX または the Ark of the Cosmos from SRZG)on his right hand,so he is supposed to put them on and take them off with his left hand( a right handed person wears bcacalets and watches on his left hand) 2)In the Sonic Colours intro,he punches Eggman with his left hand 3)Sonic is left-footed,and most left-footeds are also left-handed
He holds his sword in his right hand in Sonic and the Black knight beacause it is a Wii game,and it would be weird to control a left-handed sword スイング with a right-handed remote swing.